Securities - Maturity Distribution and Yield of Investment Securities Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Securities available-for-sale, Amount:
One year or less	$ 12,921
Over 1 through 5 years	28,378
Over 5 through 10 years	10,292
Over 10 years	3,615
Total	101,968	$ 88,550
Securities held-to-maturity, Amount:
One year or less	758
Over 1 through 5 years	4,246
Over 5 through 10 years	445
Over 10 years	395
Amortized cost	37,305	34,483
Mortgage-backed securities
Securities available-for-sale, Amount:
Without single maturity date	40,444
Securities held-to-maturity, Amount:
Mortgage-backed securities	31,461
Asset-backed securities
Securities available-for-sale, Amount:
Without single maturity date	6,318
U.S. Treasury
Securities available-for-sale, Amount:
One year or less	1,353
Over 1 through 5 years	12,422
Over 5 through 10 years	6,009
Over 10 years	3,297
Total	$ 23,081	15,431
Securities available-for-sale (Yield):
One year or less	2.58%
Over 1 through 5 years	1.53%
Over 5 through 10 years	1.69%
Over 10 years	3.11%
Total	1.86%
Securities held-to-maturity, Amount:
One year or less	$ 0
Over 1 through 5 years	2,937
Over 5 through 10 years	0
Over 10 years	0
Amortized cost	$ 2,937	3,818
Securities held-to-maturity (Yield):
One year or less	0.00%
Over 1 through 5 years	1.98%
Over 5 through 10 years	0.00%
Over 10 years	0.00%
Total	1.98%
U.S. government agencies
Securities available-for-sale, Amount:
One year or less	$ 26
Over 1 through 5 years	474
Over 5 through 10 years	1,594
Over 10 years	119
Total	$ 2,213	1,949
Securities available-for-sale (Yield):
One year or less	2.46%
Over 1 through 5 years	1.73%
Over 5 through 10 years	2.70%
Over 10 years	2.06%
Total	2.46%
Securities held-to-maturity, Amount:
One year or less	$ 0
Over 1 through 5 years	603
Over 5 through 10 years	259
Over 10 years	382
Amortized cost	$ 1,244	1,023
Securities held-to-maturity (Yield):
One year or less	0.00%
Over 1 through 5 years	1.86%
Over 5 through 10 years	2.30%
Over 10 years	2.43%
Total	2.12%
State and political subdivisions
Securities available-for-sale, Amount:
One year or less	$ 165
Over 1 through 5 years	719
Over 5 through 10 years	88
Over 10 years	13
Total	$ 985	1,044
Securities available-for-sale (Yield):
One year or less	2.93%
Over 1 through 5 years	3.29%
Over 5 through 10 years	2.58%
Over 10 years	2.32%
Total	3.15%
Securities held-to-maturity, Amount:
One year or less	$ 0
Over 1 through 5 years	3
Over 5 through 10 years	0
Over 10 years	13
Amortized cost	$ 16	$ 17
Securities held-to-maturity (Yield):
One year or less	0.00%
Over 1 through 5 years	5.68%
Over 5 through 10 years	0.00%
Over 10 years	4.76%
Total	4.92%
Other bonds, notes and debentures
Securities available-for-sale, Amount:
One year or less	$ 11,377
Over 1 through 5 years	14,763
Over 5 through 10 years	2,601
Over 10 years	186
Total	$ 28,927
Securities available-for-sale (Yield):
One year or less	1.20%
Over 1 through 5 years	0.76%
Over 5 through 10 years	0.65%
Over 10 years	1.67%
Total	0.93%
Securities held-to-maturity, Amount:
One year or less	$ 758
Over 1 through 5 years	703
Over 5 through 10 years	186
Over 10 years	0
Amortized cost	$ 1,647
Securities held-to-maturity (Yield):
One year or less	1.53%
Over 1 through 5 years	0.68%
Over 5 through 10 years	0.69%
Over 10 years	0.00%
Total	1.06%
Mortgage/ asset-backed
Securities available-for-sale, Amount:
Total	$ 46,762
Securities available-for-sale (Yield):
Total	2.53%
Securities held-to-maturity, Amount:
Amortized cost	$ 31,461
Securities held-to-maturity (Yield):
Total	2.88%
Mortgage/ asset-backed | Mortgage-backed securities
Securities available-for-sale, Amount:
Without single maturity date	$ 40,444
Securities available-for-sale (Yield):
Without single maturity date	2.50%
Securities held-to-maturity, Amount:
Mortgage-backed securities	$ 31,461
Securities held-to-maturity (Yield):
Without single maturity date	2.88%
Mortgage/ asset-backed | Asset-backed securities
Securities available-for-sale, Amount:
Without single maturity date	$ 6,318
Securities available-for-sale (Yield):
Without single maturity date	2.70%