Acquisition, Divestiture, and Discontinued Operations - Portfolio Loans at Fair Value and Related Contractual Amounts (Detail) - Trust [Member] - Discontinued Operations [Member] - Education Lending [Member] - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio contractual amount of loans	$ 3	$ 4
Portfolio fair value of loans	$ 3	$ 4