Securities - Maturity Distribution and Yield of Investment Securities Portfolio (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Securities available-for-sale, Amount:
One year or less	$ 10,299
Over 1 through 5 years	30,313
Over 5 through 10 years	20,310
Over 10 years	3,337
Total	101,007	$ 108,495
Securities held-to-maturity, Amount:
One year or less	1,748
Over 1 through 5 years	7,689
Over 5 through 10 years	4,851
Over 10 years	156
Amortized cost	56,267	47,946
Mortgage-backed securities
Securities available-for-sale, Amount:
Without single maturity date	29,232
Securities held-to-maturity, Amount:
Mortgage-backed securities	41,823
Asset-backed securities
Securities available-for-sale, Amount:
Without single maturity date	7,516
U.S. Treasury
Securities available-for-sale, Amount:
One year or less	2,159
Over 1 through 5 years	8,883
Over 5 through 10 years	12,051
Over 10 years	2,868
Total	$ 25,961	24,894
Securities available-for-sale (Yield):
One year or less	1.44%
Over 1 through 5 years	1.01%
Over 5 through 10 years	1.22%
Over 10 years	3.10%
Total	1.35%
Securities held-to-maturity, Amount:
One year or less	$ 1,653
Over 1 through 5 years	5,978
Over 5 through 10 years	2,897
Over 10 years	0
Amortized cost	$ 10,528	2,938
Securities held-to-maturity (Yield):
One year or less	1.27%
Over 1 through 5 years	0.95%
Over 5 through 10 years	1.21%
Over 10 years	0.00%
Total	1.07%
U.S. government agencies
Securities available-for-sale, Amount:
One year or less	$ 195
Over 1 through 5 years	1,720
Over 5 through 10 years	614
Over 10 years	114
Total	$ 2,643	3,853
Securities available-for-sale (Yield):
One year or less	0.21%
Over 1 through 5 years	0.95%
Over 5 through 10 years	2.00%
Over 10 years	1.91%
Total	1.17%
Securities held-to-maturity, Amount:
One year or less	$ 0
Over 1 through 5 years	862
Over 5 through 10 years	1,869
Over 10 years	148
Amortized cost	$ 2,879	2,816
Securities held-to-maturity (Yield):
One year or less	0.00%
Over 1 through 5 years	0.68%
Over 5 through 10 years	1.08%
Over 10 years	1.88%
Total	1.00%
State and political subdivisions
Securities available-for-sale, Amount:
One year or less	$ 235
Over 1 through 5 years	633
Over 5 through 10 years	1,499
Over 10 years	277
Total	$ 2,644	2,308
Securities available-for-sale (Yield):
One year or less	2.44%
Over 1 through 5 years	2.07%
Over 5 through 10 years	1.55%
Over 10 years	2.26%
Total	1.83%
Securities held-to-maturity, Amount:
One year or less	$ 1
Over 1 through 5 years	1
Over 5 through 10 years	4
Over 10 years	8
Amortized cost	$ 14	$ 15
Securities held-to-maturity (Yield):
One year or less	5.52%
Over 1 through 5 years	5.72%
Over 5 through 10 years	4.65%
Over 10 years	4.80%
Total	4.89%
Other bonds, notes and debentures
Securities available-for-sale, Amount:
One year or less	$ 7,710
Over 1 through 5 years	19,077
Over 5 through 10 years	6,146
Over 10 years	78
Total	$ 33,011
Securities available-for-sale (Yield):
One year or less	0.58%
Over 1 through 5 years	0.49%
Over 5 through 10 years	0.74%
Over 10 years	1.14%
Total	0.56%
Securities held-to-maturity, Amount:
One year or less	$ 94
Over 1 through 5 years	848
Over 5 through 10 years	81
Over 10 years	0
Amortized cost	$ 1,023
Securities held-to-maturity (Yield):
One year or less	0.45%
Over 1 through 5 years	0.70%
Over 5 through 10 years	0.59%
Over 10 years	0.00%
Total	0.66%
Mortgage/ asset-backed
Securities available-for-sale, Amount:
Total	$ 36,748
Securities available-for-sale (Yield):
Total	1.81%
Securities held-to-maturity, Amount:
Amortized cost	$ 41,823
Securities held-to-maturity (Yield):
Total	2.27%
Mortgage/ asset-backed | Mortgage-backed securities
Securities available-for-sale, Amount:
Without single maturity date	$ 29,232
Securities available-for-sale (Yield):
Without single maturity date	1.93%
Securities held-to-maturity, Amount:
Mortgage-backed securities	$ 41,823
Securities held-to-maturity (Yield):
Without single maturity date	2.27%
Mortgage/ asset-backed | Asset-backed securities
Securities available-for-sale, Amount:
Without single maturity date	$ 7,516
Securities available-for-sale (Yield):
Without single maturity date	1.38%