Patents have named inventors, which can not be companies: they have to be people.
When someone works for a company (actually because of this rule that patents are assigned to individual inventors) they agree as a condition of employment to "assign" the rights to any patents awarded to them in conjunction with performing work for the company (and sometimes not when doing so also) to said company.
So, did they company pay for filing of a patent with you as a named inventor? If so then you signed away the rights to this patent (or you did if they have their legal paperwork straight). You own it as much as I do. You might try asking them nicely for a license to use it -- they might be inclined to help you since to some extent they also need your good will for any future litigation over said patent.
Or, did someone else at the company file a patent covering something that you believe you (not them) invented? If so then prepare to open a legal can of worms.
I guess you might also be saying that the company filed a patent with or without you as a named inventor that you believe is weak (not really a patentable invention) and you're wondering if this is any different than the likely 1000 other weak patents out there covering the same "invention"?
Perhaps if you post more details on the specifics of the situation folks here can help with more accurate advice.
btw as an aside : how can someone work in this industry and _not_ know this? I'm not questioning your sincerity but I mean why isn't everyone handed a short pamphlet to read when they get into this business that explains this stuff?
Anyway, back to your situation : based on what you have said I'd suggest going to your management and asking if they would be interested in commercializing your invention as some sort of joint venture or spin-off company. They would provide your company with a license in exchange for, say, equity. In return they would get to enjoy your full and enthusiastic participation in the patent filing and defense process. Worth a try.
I'd consider filing your own provisional patent ($70), which will never see the light of day unless you continue the application in a year, but will give you the priority date.
Filing, even for a provisional, isn't trivial. To survive the decades-long process, you'll need claims and drawings from the get-go. You'll also need to scour google.com/patents to make sure it hasn't already been invented.
It won't become known for a year-plus after filing that you have this patent application, and by then hopefully the old company will have forgotten about your little technology.
But rather than being sneaky, instead of signing, consider proposing a separation agreement that basically says you're done, everybody's square and nobody's suing anybody for any reason. If they say whoa there, just say you'd prefer not to patent anything because you value your privacy. You're probably hard to replace and I'd never trade a good employee for a patent.
This gives them the choice of firing you (meaning you can file yourself with no contract ramifications) or giving you a free pass on your patent idea.
It's not a decades-long process. I've seen it be closer to 2 years.
Filing application until issue of patent is only the very beginning. You have renewal payments at 3.5, 7.5, and 11.5 years after issue. If you get a restriction requirement, you get to start the clock over each time as your single patent application blossoms into multiple, or dozens, or hundreds, if it's deemed worth the expense. Continuations-in-part and reissues can take it even further.
And that doesn't even bring litigation into the picture. Your patent can be long expired before you file an infringement lawsuit (because you can demonstrate they infringed when it was valid), and that by itself can add 10+ years, as we're seeing in many cases out of the courts.
For what it's worth, I'm on year 9 of the filing-to-issue process for my first patent.
It'll save you 10-100x that much in pain and suffering. And it always pays dividends to talk to a professional.
Odds are the company won't understand this, or that only you could make this idea work (in a sense ideas choose people, not the other way around), and the patent may end up being useless to them. It's ok. Let it go.
If your new startup does something similar, but with enough variations that you can prove doesn't meet all the claims (here is where you need the lawyer), then you're good.
Be very careful with advice like this.
Patents of different classes of claims, dependent and independent, for example. Some stand on their own, some don't. Patents encapsulate lots of claims, and violating any one of them can represent a significant IP infringement and could lead to damages.
Your advice about 'enough variations that you can prove doesn't meet...' is dangerous. What you mean to say is that, "if you are willing to spend up to and potentially beyond $100,000, a jury might make the determination that you did not infringe." Lawyers don't determine whether or not you infringe, juries do. And, the most common resolution in IP is ceasing activities or licensing, which are usually settled out of court.
I guess you don't have (such agreement).