What do you mean by idiosyncratic risk here? It seems as though you just read the word in an online forum where it was used in an context free manner. Read through investopedia.com/terms/i/idiosyncraticrisk.asp and tell me how it applies here?
To answer your question, "idiosyncratic risk" in this context means that an individual company might have problems that don't broadly apply to the rest of its industry. For example Credit Suisse might have management that is bad at running a bank, leading to repeated investment losses and regulatory actions well beyond what's normal for big multinational banks.
Look at the CS balance sheets over the past few years, or its stock price and PE ratio, or hell just Google "Credit Suisse books loss" and look at how many times they tried to stick their fingers in the wrong cookie jar. They got hit by the Hwang thing, they got hit by Greensill, and apparently they can't even accurately report how much money they're making (losing).
I wouldn't want to have any position on their equity either.
I’d also welcome any interesting further reading on the subject!
However, if you’d like to learn more about $CS and Archegos I’d recommend reading the Report put out by $CS on the topic.
Colloquially known as, “Credit Suisse Group Special Committee of the Board of Directors Report on Archegos Capital Management”
https://www.sec.gov/Archives/edgar/data/1159510/000137036821...
It’s all about the material risks Archegos posed to Credit Suisse.
CS failed to capture a number of specific risks which were intrinsic to Archegos’ specific trading strategy. I won’t go through them all but they explicitly call out “idiosyncratic risk” due to their use of equity total return swaps, baskets of them, to hide equity positions. The risk being if the components of the basket, which were may have been billed to be diversified, all the sudden begin to move violently and in sequence, it would be a material idiosyncratic risk to $CS.
A large number of these swaps from 2021 are coming due this week and next. Including likely a large number today, March 15, which is a commonly used date for expiry of EuroDollar and Forex contracts, as well as presumably equity swaps as well?
Now, what’s in those swaps? Who knows, the CFTC announced an exemption back in 2021 allowing NO REPORTING of swaps through at least the fall of this year, which has subsequently been extended through 2025. So we shall see how the dominoes fall and only after will they let us see how they were setup.