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Mortgage Servicing Rights - Changes in Fair Value of Capitalized MSRs (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Transfers And Servicing Of Financial Assets [Abstract]    
Balance at beginning of period $ 1,700 $ 1,519
Rights purchased 2 13
Rights capitalized 254 261
Changes in fair value of MSRs    
Due to fluctuations in market interest rates 127 64
Due to revised assumptions or models (6) 1
Other changes in fair value (122) (121)
Balance at end of period $ 1,955 $ 1,737