Acquisitions and Discontinued Operations - Components of Assets of Education Lending Business (Parenthetical) (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Unearned income on loans	$ 645,000,000	$ 682,000,000	$ 685,000,000
Maximum [Member] | Education Lending [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Unearned income on loans	$ 1,000,000	$ 1,000,000	$ 1,000,000