Fair Value Measurement - Schedule of Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Financial assets:
Securities available for sale	$ 39,431	$ 39,508
Securities held to maturity	24,913	23,634
Derivative Assets, Gross Amount	1,911	1,452
Financial liabilities:
Derivative Liabilities, Gross Amount	472	339
Level 1
Financial assets:
Cash and cash equivalents	6,837	7,242
Restricted cash for securitization investors	586	234
Securities held to maturity	200	0
Net loans held for investment	0	0
Loans held for sale	0	0
Interest receivable	0	0
Financial liabilities:
Non-interest bearing deposits	25,055	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	1,021	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	24,662	23,503
Net loans held for investment	0	0
Loans held for sale	591	650
Interest receivable	1,101	1,079
Financial liabilities:
Non-interest bearing deposits	0	0
Interest-bearing deposits	14,605	11,668
Securitized debt obligations	15,731	11,745
Senior and subordinated notes	21,728	19,083
Federal funds purchased and securities loaned or sold under agreements to repurchase	0	0
Other borrowings	4,316	17,275
Interest payable	198	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	209,460	207,104
Loans held for sale	0	0
Interest receivable	0	0
Financial liabilities:
Non-interest bearing deposits	0	0
Interest-bearing deposits	167,978	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	6,837	7,242
Restricted cash for securitization investors	586	234
Securities held to maturity	23,711	22,500
Net loans held for investment	208,482	203,933
Loans held for sale	566	626
Interest receivable	1,101	1,079
Financial liabilities:
Non-interest bearing deposits	25,055	25,081
Interest-bearing deposits	187,848	180,467
Securitized debt obligations	15,656	11,624
Senior and subordinated notes	21,773	18,684
Federal funds purchased and securities loaned or sold under agreements to repurchase	1,021	880
Other borrowings	4,328	17,269
Interest payable	198	254
Estimated Fair Value
Financial assets:
Cash and cash equivalents	6,837	7,242
Restricted cash for securitization investors	586	234
Securities available for sale	39,431	39,508
Securities held to maturity	24,913	23,634
Net loans held for investment	209,460	207,104
Loans held for sale	591	650
Interest receivable	1,101	1,079
Derivative Assets, Gross Amount	1,911	1,452
Retained interests in securitizations	214	221
Financial liabilities:
Non-interest bearing deposits	25,055	25,081
Interest-bearing deposits	182,583	174,074
Securitized debt obligations	15,731	11,745
Senior and subordinated notes	21,728	19,083
Federal funds purchased and securities loaned or sold under agreements to repurchase	1,021	880
Other borrowings	4,316	17,275
Interest payable	198	254
Derivative Liabilities, Gross Amount	472	339
Fair Value, Measurements, Recurring [Member]
Financial assets:
Securities available for sale	39,431	39,508
Derivative Assets, Gross Amount	1,911	1,452
Retained interests in securitizations	214	221
Financial liabilities:
Derivative Liabilities, Gross Amount	472	339
Fair Value, Measurements, Recurring [Member] | Level 1
Financial assets:
Securities available for sale	4,563	4,228
Derivative Assets, Gross Amount	2	4
Retained interests in securitizations	0	0
Financial liabilities:
Derivative Liabilities, Gross Amount	4	3
Fair Value, Measurements, Recurring [Member] | Level 2
Financial assets:
Securities available for sale	34,189	34,075
Derivative Assets, Gross Amount	1,844	1,382
Retained interests in securitizations	0	0
Financial liabilities:
Derivative Liabilities, Gross Amount	433	293
Fair Value, Measurements, Recurring [Member] | Level 3
Financial assets:
Securities available for sale	679	1,205
Derivative Assets, Gross Amount	65	66
Retained interests in securitizations	214	221
Financial liabilities:
Derivative Liabilities, Gross Amount	35	43
Fair Value, Measurements, Recurring [Member] | Carrying Amount
Financial assets:
Securities available for sale	39,431	39,508
Derivative Assets, Gross Amount	1,911	1,452
Retained interests in securitizations	214	221
Financial liabilities:
Derivative Liabilities, Gross Amount	$ 472	$ 339