Acquisitions and Discontinued Operations - Consolidated Trusts' Assets and Liabilities at Fair Value and Contractual Values (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust loans accounted for at fair value	$ 187	$ 191	$ 143
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust fair value of securities			1,796
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			1,893
Trust fair value of other assets			19
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	186	192	136
Trust contractual amount of loans			1,889
Trust contractual amount of other assets			19
Trust contractual amount of securities			1,904
Trust contractual amount of other liabilities			20
Portfolio fair value of loans	187	191	143
Trust loans accounted for at fair value			1,893
Trust fair value of other assets			19
Trust fair value of securities			1,796
Trust fair value of other liabilities			$ 20