Acquisition, Divestiture, and Discontinued Operations - Additional Information (Detail)						3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 01, 2016 USD ($)	Dec. 08, 2015 USD ($)	Oct. 30, 2015 $ / shares shares	Oct. 29, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2016 USD ($) Trust	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) SecurityLoan Trust TDRs	Sep. 30, 2015 USD ($) SecurityLoan TDR	Dec. 31, 2015 USD ($) SecurityLoan TDR	Sep. 09, 2016 USD ($) Branch	Jun. 30, 2015 USD ($) Trust	Jan. 01, 2010 Trust
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust assets removed						$ 1,654,000,000	$ 2,086,000,000	$ 1,654,000,000	$ 2,086,000,000	$ 1,846,000,000
Number of TDRs | SecurityLoan								4,643	4,659	4,622
Allowance for loan held as investment						865,000,000	790,000,000	$ 865,000,000	$ 790,000,000	$ 796,000,000
Portfolio loans held for sale at fair value						0	0	0	0	0
Number of outstanding securitizations trusts | Trust												3
Proceeds from loans sold								5,181,000,000	5,362,000,000
Gain on sales of loans								92,000,000	75,000,000
Loans at fair value						3,000,000		3,000,000		4,000,000
Sale of loan										$ 162,000,000
Income (loss) from discontinued operations, net of taxes						$ 1,000,000	(3,000,000)	$ 5,000,000	$ 5,000,000
First Niagara Financial Group, Inc. [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total consideration	$ 4,000,000,000
Acquiree shares equivalent of acquirer common stock | shares			0.680
Cash payable per acquiree common stock | $ / shares			$ 2.30
Number of branches sold | Branch											18
Loans included in divestiture											$ 439,000,000
Deposits included in divestiture											$ 1,600,000,000
Pacific Crest Securities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of outstanding securitization trusts | Trust						3		3
Proceeds from sale of residual interests					$ 57,000,000
Number of continuing securitization trusts | Trust						8		8
Estimated fair value of the assets and deposits acquired					1,000,000
Education Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of TDRs								2,145	1,845	1,901
Investment recorded						$ 22,000,000	20,000,000	$ 22,000,000	$ 20,000,000	$ 21,000,000
Allowance for loan held as investment						2,000,000	2,000,000	2,000,000	2,000,000	2,000,000
Government Guaranteed Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from loans sold				$ 117,000,000
Private Loans [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from loans sold		$ 45,000,000
Gain on sales of loans		$ 1,000,000
Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans held for sale at fair value												$ 179,000,000
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
Trust liabilities removed					$ 1,600,000,000
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loans included in divestiture						1,610,000,000	1,868,000,000	1,610,000,000	1,868,000,000	1,800,000,000
Number of outstanding securitization trusts | Trust													10
Trust assets removed						1,654,000,000	2,086,000,000	1,654,000,000	2,086,000,000	1,846,000,000
Portfolio loans held for sale at fair value						0	169,000,000	0	169,000,000	0
Loans at fair value						3,000,000	0	3,000,000	0	4,000,000
Income (loss) from discontinued operations, net of taxes						1,000,000	(3,000,000)	5,000,000	5,000,000
Discontinued Operations [Member] | Austin Capital Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Trust assets removed						15,000,000	15,000,000	15,000,000	15,000,000	$ 15,000,000
Income (loss) from discontinued operations, net of taxes						$ 0	$ 0	$ 0	$ 0