I'm sensitive to the person who noted that this is probably the wrong board for this discussion but....
I'm far from a complete expert on this but I was a risk manager for one of the big banks for a long time so I'm fairly familiar with the workings. And from that I can say that as an investor you can NEVER know what the true risk is at any bank. You know how much the exposure is on a single day and the nature of it but the details are sooooo much more complicated. Plus there's the fraud risk (just ask UBS).
Rarely do banks like WFC or BAC use credit derivatives for outright positioning purposes. They are generally purchased as a hedge or in an arbitrage situation. This is probably even more true now that the trading books are being minimized and shut down. For example, WFC might lend $100m to General Motors. This is usually done, however, not because the loan is such a great asset but because they have to do it to win more lucrative FX, asset management or other business from GM. Not wanting the GM exposure they will buy $100m (or less) of CDS on GM as a hedge. If the hedge is an accurate one, this not only gives them protection against default of GM but also means they don't have to hold as much capital against the loan so it's win-win. In an arbitrage situation, they might buy $100m of GM bonds that pay a spread of 200bps and sell CDS credit protection on $100m that generates an income of 220bps - thereby keeping 20bps/year. The notionals are big but the risk is actually fairly minimal.
On the counterparty risk: there are generally 3 sellers of CDS (ie providing the protection) - banks, hedge funds, insurance companies. These days, virtually all of the trading of derivatives between these parties is done on a fully netted and collateralized basis. That means that every day they mark all the positions between them, net off all the +'s and -'s and then whoever is out-of-the-money posts collateral in that amount to the other. The AIG situation where you are uncollateralized unless the counterparty is downgraded is now a thing of the past. For hedge funds, they generally have to post excess collateral
From a capital perspective, any exposure that's fully collateralized gets a far reduced (or zero) capital requirement. The capital requirements on derivatives are extremely complicated and not all that well developed (they try hard but it's such a broad category it's tough to accurately capture it).
This is a VERY simplified discussion but I hope it points you in the right direction. The reality is about 100x more complicated and intricate. I reiterate though that nobody on the outside will ever be able to fully understand the amount, nature and risks of the exposure at the banks. This is true not just of derivatives but also the general loan/securities books. BB is a genius and probably has done more work on it than anyone but, in my view, with banks you're really betting on the big picture and management.