Well...yes and no. Big banks also routinely keep lots of off-balance sheet assets to expand their lending. For instance, right now Citigroup has about $1.1 Trillion of said off-the-books assets lurking out there.
The last round of them pulling those types of assets back on their balance sheets, in the past 10 months or so, amounted to $100 billion, generating $7 billion in losses from credit/liquidity guarantees. CDOs, etc... anyone?
In a nutshell, you really can't trust what they tell you they have and don't have, on their filings and public books. It's simply not the entire story, though it's perfectly legal in todays world. Remember Enron?
I get a lot of my financial info from Mish's discussions on his website/blog globaleconomicanalysis.blogspot.com. The above specifics on Citigroup are a paraphrase of info he's provided. He's the man!

@llecs wrote:
I suppose if your were to look at the survivability of a martgage lender, you'd want to look at their books, esp. their balance sheet and their P&L (to see how much they wrote off). Banks and CUs are required to publish this info and is easily found on their website. If a bank is publicly traded, you can find it on their quarterly report online.