Acquisitions and Discontinued Operations - Change in Fair Values of Level 3 Portfolio Loans Held for Sale, Portfolio Loans, and Consolidated Education Loan Securitization Trusts (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loans held for sale	$ 0	$ 0	$ 0	$ 0	$ 0
Discontinued Operations [Member] | Education Lending [Member] | Portfolio Student Loans Held For Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance	179,000,000
Gains (losses) recognized in earnings	(4,000,000)		(4,000,000)
Settlements	(6,000,000)		(6,000,000)
Loans held for sale	179,000,000		179,000,000
Ending Balance	169,000,000		169,000,000
Discontinued Operations [Member] | Education Lending [Member] | Portfolio Student Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		209,000,000	191,000,000	147,000,000
Gains (losses) recognized in earnings			1,000,000	(4,000,000)
Purchases				74,000,000
Settlements		(8,000,000)	(13,000,000)	(16,000,000)
Loans held for sale	$ (179,000,000)		$ (179,000,000)
Ending Balance		201,000,000		201,000,000
Discontinued Operations [Member] | Education Lending [Member] | Trust Student Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		1,711,000,000		1,960,000,000
Gains (losses) recognized in earnings				(34,000,000)
Sales				(74,000,000)
Settlements		(61,000,000)		(202,000,000)
Transfers out due to deconsolidation		(1,650,000,000)		(1,650,000,000)
Discontinued Operations [Member] | Education Lending [Member] | Trust Other Assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		19,000,000		20,000,000
Settlements				(1,000,000)
Transfers out due to deconsolidation		(19,000,000)		(19,000,000)
Discontinued Operations [Member] | Education Lending [Member] | Trust Other Liabilities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		17,000,000		20,000,000
Settlements				(3,000,000)
Transfers out due to deconsolidation		(17,000,000)		(17,000,000)
Discontinued Operations [Member] | Trust Securities [Member] | Education Lending [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		1,660,000,000		1,834,000,000
Gains (losses) recognized in earnings				33,000,000
Settlements		(71,000,000)		(278,000,000)
Transfers out due to deconsolidation		$ (1,589,000,000)		$ (1,589,000,000)