Acquisitions and Discontinued Operations - Consolidated Trusts' Assets and Liabilities at Fair Value and Contractual Values (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust loans accounted for at fair value	$ 148	$ 157	$ 71
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	2,135	2,369	2,513
Trust fair value of securities	2,016	2,159	2,310
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	142
Trust contractual amount of loans	2,190
Trust contractual amount of other assets	23
Trust contractual amount of securities	2,200
Trust contractual amount of other liabilities	21
Portfolio fair value of loans	148
Trust loans accounted for at fair value	2,135	2,369	2,513
Trust fair value of other assets	23
Trust fair value of securities	2,016	2,159	2,310
Trust fair value of other liabilities	$ 21