Securities - Maturity Distribution and Yield of Investment Securities Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Securities available-for-sale, Amount:
One year or less	$ 16,165
Over 1 through 5 years	30,751
Over 5 through 10 years	11,448
Over 10 years	3,965
Total	109,243	$ 88,550
Securities held-to-maturity, Amount:
One year or less	1,092
Over 1 through 5 years	4,704
Over 5 through 10 years	596
Over 10 years	507
Amortized cost	46,096	34,483
Mortgage-backed securities
Securities available-for-sale, Amount:
Without single maturity date	39,327
Securities held-to-maturity, Amount:
Mortgage-backed securities	39,197
Asset-backed securities
Securities available-for-sale, Amount:
Without single maturity date	7,587
U.S. Treasury
Securities available-for-sale, Amount:
One year or less	4,405
Over 1 through 5 years	10,546
Over 5 through 10 years	6,517
Over 10 years	3,265
Total	$ 24,733	15,431
Securities available-for-sale (Yield):
One year or less	1.11%
Over 1 through 5 years	1.28%
Over 5 through 10 years	1.57%
Over 10 years	3.11%
Total	1.57%
Securities held-to-maturity, Amount:
One year or less	$ 785
Over 1 through 5 years	2,503
Over 5 through 10 years	0
Over 10 years	0
Amortized cost	$ 3,288	3,818
Securities held-to-maturity (Yield):
One year or less	1.43%
Over 1 through 5 years	1.90%
Over 5 through 10 years	0.00%
Over 10 years	0.00%
Total	1.79%
U.S. government agencies
Securities available-for-sale, Amount:
One year or less	$ 25
Over 1 through 5 years	1,866
Over 5 through 10 years	1,468
Over 10 years	119
Total	$ 3,478	1,949
Securities available-for-sale (Yield):
One year or less	2.55%
Over 1 through 5 years	0.85%
Over 5 through 10 years	2.51%
Over 10 years	2.06%
Total	1.60%
Securities held-to-maturity, Amount:
One year or less	$ 0
Over 1 through 5 years	1,253
Over 5 through 10 years	564
Over 10 years	449
Amortized cost	$ 2,266	1,023
Securities held-to-maturity (Yield):
One year or less	0.00%
Over 1 through 5 years	0.82%
Over 5 through 10 years	1.13%
Over 10 years	2.28%
Total	1.19%
State and political subdivisions
Securities available-for-sale, Amount:
One year or less	$ 549
Over 1 through 5 years	570
Over 5 through 10 years	223
Over 10 years	348
Total	$ 1,690	1,044
Securities available-for-sale (Yield):
One year or less	1.49%
Over 1 through 5 years	3.16%
Over 5 through 10 years	1.88%
Over 10 years	2.22%
Total	2.26%
Securities held-to-maturity, Amount:
One year or less	$ 0
Over 1 through 5 years	2
Over 5 through 10 years	0
Over 10 years	13
Amortized cost	$ 15	$ 17
Securities held-to-maturity (Yield):
One year or less	0.00%
Over 1 through 5 years	5.66%
Over 5 through 10 years	0.00%
Over 10 years	4.76%
Total	4.91%
Other bonds, notes and debentures
Securities available-for-sale, Amount:
One year or less	$ 11,186
Over 1 through 5 years	17,769
Over 5 through 10 years	3,240
Over 10 years	233
Total	$ 32,428
Securities available-for-sale (Yield):
One year or less	0.44%
Over 1 through 5 years	0.61%
Over 5 through 10 years	0.56%
Over 10 years	0.62%
Total	0.55%
Securities held-to-maturity, Amount:
One year or less	$ 307
Over 1 through 5 years	946
Over 5 through 10 years	32
Over 10 years	45
Amortized cost	$ 1,330
Securities held-to-maturity (Yield):
One year or less	1.99%
Over 1 through 5 years	0.67%
Over 5 through 10 years	0.92%
Over 10 years	0.35%
Total	0.96%
Mortgage/ asset-backed
Securities available-for-sale, Amount:
Total	$ 46,914
Securities available-for-sale (Yield):
Total	2.10%
Securities held-to-maturity, Amount:
Amortized cost	$ 39,197
Securities held-to-maturity (Yield):
Total	2.57%
Mortgage/ asset-backed | Mortgage-backed securities
Securities available-for-sale, Amount:
Without single maturity date	$ 39,327
Securities available-for-sale (Yield):
Without single maturity date	2.18%
Securities held-to-maturity, Amount:
Mortgage-backed securities	$ 39,197
Securities held-to-maturity (Yield):
Without single maturity date	2.57%
Mortgage/ asset-backed | Asset-backed securities
Securities available-for-sale, Amount:
Without single maturity date	$ 7,587
Securities available-for-sale (Yield):
Without single maturity date	1.72%