Acquisitions and Discontinued Operations - Portfolio Loans Held for Sale at Fair Value and Portfolio Loans at Fair Value and Related Contractual Amounts (Detail) - Discontinued Operations [Member] - Trust [Member] - Education Lending [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio contractual amount of loans held for sale	$ 173
Portfolio contractual amount of loans		$ 192	$ 199
Portfolio fair value of loans held for sale	$ 169
Portfolio fair value of loans		$ 191	$ 201