Acquisitions and Discontinued Operations - Additional Information (Detail)		3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 29, 2015 USD ($)	Sep. 30, 2015 USD ($) Trust	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2015 USD ($) SecurityLoan Trust TDRs	Sep. 30, 2014 USD ($) SecurityLoan Trust	Dec. 31, 2014 USD ($) SecurityLoan	Dec. 31, 2009 Trust
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Estimated goodwill		$ 1,060,000,000			$ 1,060,000,000	$ 1,051,000,000	$ 1,057,000,000
Trust assets removed		2,086,000,000			2,086,000,000	2,421,000,000	2,324,000,000
Discontinued liabilities		0			0	3,000,000	3,000,000
Allowance for loan held as investment		$ 790,000,000			$ 790,000,000	$ 804,000,000	$ 794,000,000
Number of TDRs | SecurityLoan					4,659	4,730	4,776
Loans at fair value						$ 201,000,000	$ 191,000,000
Number of outstanding securitizations trusts | Trust		3			3
Loans held for sale at fair value		$ 0			$ 0	0	0
Proceeds from loans sold					$ 5,362,000,000	$ 2,832,000,000
Cash portion in gain				$ 6,000,000
Pacific Crest Securities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business acquisition, date of acquisition					Sep. 03, 2014
Business acquisition intangible asset							13,000,000
Estimated goodwill							78,000,000
Increase in goodwill		3,000,000
Number of outstanding securitization trusts | Trust						3
Proceeds from sale of residual interests						$ 57,000,000
After tax loss recognized						25,000,000
Estimated fair value of the assets and deposits acquired						$ 1,000,000
Number of continuing securitization trusts | Trust						8
Subsequent Event [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from loans sold	$ 117,000,000
Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loans held for sale at fair value		173,000,000			$ 173,000,000
Trust [Member] | Pacific Crest Securities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust assets removed						$ 1,700,000,000
Discontinued liabilities						1,600,000,000
Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loans held for sale at fair value		169,000,000			169,000,000
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of outstanding securitization trusts | Trust								10
Trust assets removed		2,086,000,000			2,086,000,000	2,421,000,000	2,324,000,000
Discontinued liabilities						3,000,000	3,000,000
Economic interest or risk of loss associated with education loan securitization trusts		0			0
Fair value of loans and securities in securitization trusts after tax loss			$ 22,000,000
Discontinued Operations [Member] | Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loans at fair value						$ 201,000,000	$ 191,000,000
Loans held for sale at fair value		169,000,000			169,000,000
Education Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investment recorded		20,000,000			20,000,000
Allowance for loan held as investment		$ 2,000,000			$ 2,000,000
Number of TDRs | TDRs					1,845