Acquisitions and Discontinued Operations - Consolidated Trusts' Assets and Liabilities at Fair Value and Contractual Values (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust loans accounted for at fair value	$ 151	$ 157	$ 73
Segment, Discontinued Operations [Member] | Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust loans accounted for at fair value	2,317	2,369	2,580
Trust fair value of securities	2,118	2,159	2,373
Segment, Discontinued Operations [Member] | Trust [Member] | Education Lending [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio contractual amount of loans	145
Trust contractual amount of loans	2,269
Trust contractual amount of other assets	24
Trust contractual amount of securities	2,288
Trust contractual amount of other liabilities	21
Portfolio fair value of loans	151
Trust loans accounted for at fair value	2,317	2,369	2,580
Trust fair value of other assets	24
Trust fair value of securities	2,118	2,159	2,373
Trust fair value of other liabilities	$ 21