Bank of America is doing its best to calm fears about potential losses from having to buy back dodgy mortgages. The details executives laid out on the firm's third-quarter earnings call were welcome. They were also necessary: the bank's stock plummeted some 14 percent late last week after shareholders were spooked by a two-month-old presentation by hedge fund Branch Hill Capital estimating repurchases would cost the bank $74 billion. But B of A's helpful disclosures fell a tad short.

That's not the bank's fault. Unlike three years ago, when the likes of Citigroup and Merrill Lynch didn't even know how much subprime mortgage guff they had stuffed into their balance sheets, B of A knows its overall exposure. But working out mortgage repurchases is a time-consuming zero-sum game. The current owners want to foist as many poorly performing loans as possible back on the originators at face value, but must prove that the loans didn't meet the promised representations and warranties about lending standards. That requires deep dives into individual loan documents and much back and forth.

B of A's most useful information dealt with loan repurchases from Fannie Mae and Freddie Mac. There's a history of cooperation and only one party on either side of the negotiations. B of A reckons it's about two-thirds done with the process and its data imply total losses of no more than $6 billion — less than a third of what Branch Hill was predicting.

But it's much trickier estimating how much it will have to buy back from monolines and private mortgage bond investors, which collectively insured or bought $910 billion of B of A's mortgages between 2004 and 2008. Lines of communication with some monolines are poor — a couple are suing the bank. And it's much harder for private investors to challenge lenders: representations and warranties aren't as strict as for loans sold to Fannie and Freddie, and a quarter of investors in a private mortgage bond must band together just to have the right to start the conversation.

Considering how off-base Branch Hill's estimates on agency losses appear, there's a good chance the $50 billion the fund reckoned privately owned loans and bonds will cost B of A is over the top, too. But for as long as B of A is unable to give any guidance, there's a risk the fear will linger.

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