It's like a loss you would make if you bought a stock for $20 and sold it for $18. Someone else buys it for $18 and it might go up or down from there. Rosneft got their money back when they initially sold the stock to the market.

I think BP is a UK company and I'm not that familiar with the specifics of their accounting system. In the disclosure they say they considered that they had "significant influence" which is an IFRS accounting term. That would mean (as simply as I can explain it) that the initial purchase is recorded at cost with their share of Rosnefts profits and dividends recorded against that holding (along with a large bundle of other accounting details).

> Were the shares that BP held not marked to market already?

No, they were treated using the equity method:

https://en.wikipedia.org/wiki/Equity_method

In accounting terms, if they already recognized the asset at x value, or paid x amount for it, any amount less than x is considered a loss.

Depending on what goes on here market wise before they can exit their ownership stake (if they haven’t already!) is what will decide how big a loss this is.