Off-balance Sheet Lending-related Financial Instruments, Guarantees, and Other Commitments - Standby Letters of Credit and Other Financial Guarantees (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Standby letters of credit and other financial guarantees and other letters of credit [Abstract]
Total lending-related commitments	$ 1,048,674	$ 991,482	$ 999,817
Total carrying value	1,511	1,512
Standby letters of credit and other financial guarantees
Standby letters of credit and other financial guarantees and other letters of credit [Abstract]
Investment-grade	25,038	28,492
Noninvestment-grade	6,710	6,734
Total lending-related commitments	31,748	35,226
Allowance for lending-related commitments	171	192
Guarantee liability	414	444
Total carrying value	585	636
Commitments with collateral	16,074	17,421
Other letters of credit
Standby letters of credit and other financial guarantees and other letters of credit [Abstract]
Investment-grade	2,507	2,646
Noninvestment-grade	1,076	1,066
Total lending-related commitments	3,583	3,712
Allowance for lending-related commitments	7	3
Guarantee liability	0	0
Total carrying value	7	3
Commitments with collateral	$ 559	$ 878