Fair Value Measurement - Schedule of Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Financial assets:
Securities available for sale	$ 39,061	$ 39,508
Securities held to maturity	25,317	23,634
Derivative Assets, Gross Amount	1,518	1,452
Financial liabilities:
Derivative Liabilities, Gross Amount	520	339
Level 1
Financial assets:
Cash and cash equivalents	8,023	7,242
Restricted cash for securitization investors	1,017	234
Securities held to maturity	198	0
Net loans held for investment	0	0
Loans held for sale	0	0
Interest receivable	0	0
Financial liabilities:
Non-interest bearing deposits	25,847	25,081
Interest-bearing deposits	0	0
Securitized debt obligations	0	0
Senior and subordinated notes	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase	981	880
Other borrowings	0	0
Interest payable	0	0
Level 2
Financial assets:
Cash and cash equivalents	0	0
Restricted cash for securitization investors	0	0
Securities held to maturity	25,068	23,503
Net loans held for investment	0	0
Loans held for sale	860	650
Interest receivable	1,189	1,079
Financial liabilities:
Non-interest bearing deposits	0	0
Interest-bearing deposits	15,848	11,668
Securitized debt obligations	16,225	11,745
Senior and subordinated notes	22,062	19,083
Federal funds purchased and securities loaned or sold under agreements to repurchase	0	0
Other borrowings	20,134	17,275
Interest payable	299	254
Level 3
Financial assets:
Cash and cash equivalents	0	0
Restricted cash for securitization investors	0	0
Securities held to maturity	51	131
Net loans held for investment	222,007	207,104
Loans held for sale	73	0
Interest receivable	0	0
Financial liabilities:
Non-interest bearing deposits	0	0
Interest-bearing deposits	169,227	162,406
Securitized debt obligations	0	0
Senior and subordinated notes	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase	0	0
Other borrowings	0	0
Interest payable	0	0
Carrying Amount
Financial assets:
Cash and cash equivalents	8,023	7,242
Restricted cash for securitization investors	1,017	234
Securities held to maturity	24,619	22,500
Net loans held for investment	224,721	203,933
Loans held for sale	904	626
Interest receivable	1,189	1,079
Financial liabilities:
Non-interest bearing deposits	25,847	25,081
Interest-bearing deposits	191,874	180,467
Securitized debt obligations	16,166	11,624
Senior and subordinated notes	21,837	18,684
Federal funds purchased and securities loaned or sold under agreements to repurchase	981	880
Other borrowings	20,131	17,269
Interest payable	299	254
Estimated Fair Value
Financial assets:
Cash and cash equivalents	8,023	7,242
Restricted cash for securitization investors	1,017	234
Securities held to maturity	25,317	23,634
Net loans held for investment	222,007	207,104
Loans held for sale	933	650
Interest receivable	1,189	1,079
Financial liabilities:
Non-interest bearing deposits	25,847	25,081
Interest-bearing deposits	185,075	174,074
Securitized debt obligations	16,225	11,745
Senior and subordinated notes	22,062	19,083
Federal funds purchased and securities loaned or sold under agreements to repurchase	981	880
Other borrowings	20,134	17,275
Interest payable	299	254
Fair Value, Measurements, Recurring [Member]
Financial assets:
Securities available for sale	39,061	39,508
Derivative Assets, Gross Amount	1,518	1,452
Retained interests in securitizations	211	221
Financial liabilities:
Derivative Liabilities, Gross Amount	520	339
Fair Value, Measurements, Recurring [Member] | Level 1
Financial assets:
Securities available for sale	5,015	4,228
Derivative Assets, Gross Amount	2	4
Retained interests in securitizations	0	0
Financial liabilities:
Derivative Liabilities, Gross Amount	2	3
Fair Value, Measurements, Recurring [Member] | Level 2
Financial assets:
Securities available for sale	33,431	34,075
Derivative Assets, Gross Amount	1,459	1,382
Retained interests in securitizations	0	0
Financial liabilities:
Derivative Liabilities, Gross Amount	491	293
Fair Value, Measurements, Recurring [Member] | Level 3
Financial assets:
Securities available for sale	615	1,205
Derivative Assets, Gross Amount	57	66
Retained interests in securitizations	211	221
Financial liabilities:
Derivative Liabilities, Gross Amount	$ 27	$ 43