Acquisition, Divestiture, and Discontinued Operations - Portfolio Loans at Fair Value and Related Contractual Amounts (Detail) - Discontinued Operations [Member] - Trust [Member] - Education Lending [Member] - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio contractual amount of loans	$ 2	$ 3
Portfolio fair value of loans	$ 2	$ 3