Fair Value Measurement - Schedule of Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Securities held to maturity	$ 29,437	$ 26,196
Level 1
Financial assets:
Cash and cash equivalents	4,458	4,185
Restricted cash for securitization investors	312	2,517
Securities held to maturity	200	199
Net loans held for investment	0	0
Loans held for sale	0	0
Interest receivable	0	0
Other investments	0	0
Financial liabilities:
Deposits	26,404	25,502
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
Level 2
Financial assets:
Cash and cash equivalents	9,582	5,791
Restricted cash for securitization investors	0	0
Securities held to maturity	29,217	25,962
Net loans held for investment	0	0
Loans held for sale	949	1,038
Interest receivable	1,536	1,351
Other investments	1,680	2,020
Financial liabilities:
Deposits	217,328	211,580
Securitized debt obligations	20,122	18,920
Senior and subordinated notes	31,392	23,774
Federal funds purchased and securities loaned or sold under agreements to repurchase	576	992
Other borrowings	8,892	17,180
Interest payable	413	327
Level 3
Financial assets:
Cash and cash equivalents	0	0
Restricted cash for securitization investors	0	0
Securities held to maturity	20	35
Net loans held for investment	251,468	242,935
Loans held for sale	3	0
Interest receivable	0	0
Other investments	9	9
Financial liabilities:
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	0	0
Other borrowings	0	0
Interest payable	0	0
Carrying Value
Financial assets:
Cash and cash equivalents	14,040	9,976
Restricted cash for securitization investors	312	2,517
Securities held to maturity	28,984	25,712
Net loans held for investment	246,971	239,083
Loans held for sale	971	1,043
Interest receivable	1,536	1,351
Other investments	1,689	2,029
Financial liabilities:
Deposits	243,702	236,768
Securitized debt obligations	20,010	18,826
Senior and subordinated notes	30,755	23,431
Federal funds purchased and securities loaned or sold under agreements to repurchase	576	992
Other borrowings	8,892	17,211
Interest payable	413	327
Estimated Fair Value
Financial assets:
Cash and cash equivalents	14,040	9,976
Restricted cash for securitization investors	312	2,517
Securities held to maturity	29,437	26,196
Net loans held for investment	251,468	242,935
Loans held for sale	952	1,038
Interest receivable	1,536	1,351
Other investments	1,689	2,029
Financial liabilities:
Deposits	243,732	237,082
Securitized debt obligations	20,122	18,920
Senior and subordinated notes	31,392	23,774
Federal funds purchased and securities loaned or sold under agreements to repurchase	576	992
Other borrowings	8,892	17,180
Interest payable	$ 413	$ 327