Acquisitions and Discontinued Operations - Change in Fair Values of Level 3 Consolidated Education Loan Securitization Trusts and Portfolio Loans (Detail) (Discontinued Operations [Member], Education Lending [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Portfolio Student Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance	$ 191	$ 147
Gains (losses) recognized in earnings	3
Purchases	0	0
Sales	0	0
Settlements	(7)	(4)
Ending Balance	187	143
Trust Student Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		1,960
Gains (losses) recognized in earnings		2
Purchases	0	0
Sales	0	0
Settlements		(69)
Ending Balance		1,893
Trust Other Assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		20
Purchases	0	0
Sales	0	0
Settlements		(1)
Ending Balance		19
Trust Securities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		1,834
Gains (losses) recognized in earnings		16
Purchases	0	0
Sales	0	0
Settlements		(54)
Ending Balance		1,796
Trust Other Liabilities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Beginning Balance		20
Purchases	0	0
Sales	0	0
Ending Balance		$ 20