Securities - Maturities of Debt Securities Carried at Fair Value and Held-to-maturity Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Held-to-maturity, Maturity, Amortized Cost, Net [Abstract]
Due in One Year or Less, Amount	$ 354
Due in One Year or Less, Yield	3.96%
Due after One Year through Five Years, Amount	$ 50
Due after One Year through Five Years, Yield	3.80%
Due after Five Years through Ten Years, Amount	$ 1,112
Due after Five Years though Ten Years, Yield	2.53%
Due after Ten Years, Amount	$ 188,736
Due after Ten Years, Yield	3.24%
Amortized Cost	$ 190,252	$ 203,652
Total, Yield	3.24%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Debt securities carried at fair value	$ 254,342	238,101
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less, Amount	354
Due after One Year through Five Years, Amount	50
Due after Five Years though Ten Years, Amount	1,139
Due after Ten Years, Amount	192,731
Debt Securities, Held-to-maturity, Fair Value	194,274	$ 200,435
Mortgage-backed securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due in One Year or Less, Yield	0.00%
Due after One Year through Five Years, Amount	$ 3,446
Due after One Year through Five Years, Yield	2.28%
Due after Five Years though Ten Years, Amount	$ 11,854
Due after Five Years though Ten Years, Yield	2.57%
Due after Ten Years, Amount	$ 140,960
Due after Ten Years, Yield	3.44%
Total, Amount	$ 156,260
Total, Yield	3.35%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due after One Year through Five Years, Amount	3,484
Due after Five Years though Ten Years, Amount	12,168
Due after Ten Years, Amount	142,435
Debt securities carried at fair value	158,087
Agency
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due in One Year or Less, Yield	0.00%
Due after One Year through Five Years, Amount	$ 120
Due after One Year through Five Years, Yield	2.37%
Due after Five Years though Ten Years, Amount	$ 1,715
Due after Five Years though Ten Years, Yield	2.42%
Due after Ten Years, Amount	$ 132,139
Due after Ten Years, Yield	3.29%
Total, Amount	$ 133,974
Total, Yield	3.28%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due after One Year through Five Years, Amount	120
Due after Five Years though Ten Years, Amount	1,719
Due after Ten Years, Amount	133,111
Debt securities carried at fair value	134,950
Agency-collateralized mortgage obligations
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due in One Year or Less, Yield	0.00%
Due after One Year through Five Years, Amount	$ 0
Due after One Year through Five Years, Yield	0.00%
Due after Five Years though Ten Years, Amount	$ 27
Due after Five Years though Ten Years, Yield	2.56%
Due after Ten Years, Amount	$ 4,850
Due after Ten Years, Yield	3.17%
Total, Amount	$ 4,877
Total, Yield	3.17%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due after One Year through Five Years, Amount	0
Due after Five Years though Ten Years, Amount	28
Due after Ten Years, Amount	4,934
Debt securities carried at fair value	4,962
Commercial
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due in One Year or Less, Yield	0.00%
Due after One Year through Five Years, Amount	$ 3,326
Due after One Year through Five Years, Yield	2.28%
Due after Five Years though Ten Years, Amount	$ 10,100
Due after Five Years though Ten Years, Yield	2.60%
Due after Ten Years, Amount	$ 888
Due after Ten Years, Yield	2.99%
Total, Amount	$ 14,314
Total, Yield	2.55%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due after One Year through Five Years, Amount	3,364
Due after Five Years though Ten Years, Amount	10,395
Due after Ten Years, Amount	931
Debt securities carried at fair value	14,690
Non-agency residential
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due in One Year or Less, Yield	0.00%
Due after One Year through Five Years, Amount	$ 0
Due after One Year through Five Years, Yield	0.00%
Due after Five Years though Ten Years, Amount	$ 12
Due after Five Years though Ten Years, Yield	0.00%
Due after Ten Years, Amount	$ 3,083
Due after Ten Years, Yield	10.49%
Total, Amount	$ 3,095
Total, Yield	10.46%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 0
Due after One Year through Five Years, Amount	0
Due after Five Years though Ten Years, Amount	26
Due after Ten Years, Amount	3,459
Debt securities carried at fair value	3,485
U.S. Treasury and agency securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 482
Due in One Year or Less, Yield	0.33%
Due after One Year through Five Years, Amount	$ 36,446
Due after One Year through Five Years, Yield	1.65%
Due after Five Years though Ten Years, Amount	$ 18,803
Due after Five Years though Ten Years, Yield	2.50%
Due after Ten Years, Amount	$ 18
Due after Ten Years, Yield	2.54%
Total, Amount	$ 55,749
Total, Yield	1.92%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 483
Due after One Year through Five Years, Amount	36,629
Due after Five Years though Ten Years, Amount	19,820
Due after Ten Years, Amount	18
Debt securities carried at fair value	56,950
Non-U.S. securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 18,097
Due in One Year or Less, Yield	1.07%
Due after One Year through Five Years, Amount	$ 762
Due after One Year through Five Years, Yield	1.50%
Due after Five Years though Ten Years, Amount	$ 11
Due after Five Years though Ten Years, Yield	4.38%
Due after Ten Years, Amount	$ 101
Due after Ten Years, Yield	6.24%
Total, Amount	$ 18,971
Total, Yield	1.12%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 18,099
Due after One Year through Five Years, Amount	772
Due after Five Years though Ten Years, Amount	12
Due after Ten Years, Amount	104
Debt securities carried at fair value	18,987
Other taxable securities, substantially all asset-backed securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 520
Due in One Year or Less, Yield	3.36%
Due after One Year through Five Years, Amount	$ 2,175
Due after One Year through Five Years, Yield	3.20%
Due after Five Years though Ten Years, Amount	$ 436
Due after Five Years though Ten Years, Yield	3.29%
Due after Ten Years, Amount	$ 675
Due after Ten Years, Yield	3.61%
Total, Amount	$ 3,806
Total, Yield	3.31%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 527
Due after One Year through Five Years, Amount	2,214
Due after Five Years though Ten Years, Amount	463
Due after Ten Years, Amount	682
Debt securities carried at fair value	3,886
Total taxable securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 19,099
Due in One Year or Less, Yield	1.12%
Due after One Year through Five Years, Amount	$ 42,829
Due after One Year through Five Years, Yield	1.78%
Due after Five Years though Ten Years, Amount	$ 31,104
Due after Five Years though Ten Years, Yield	2.54%
Due after Ten Years, Amount	$ 141,754
Due after Ten Years, Yield	3.44%
Total, Amount	$ 234,786
Total, Yield	2.83%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 19,109
Due after One Year through Five Years, Amount	43,099
Due after Five Years though Ten Years, Amount	32,463
Due after Ten Years, Amount	143,239
Debt securities carried at fair value	237,910
Tax-exempt securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 724
Due in One Year or Less, Yield	1.98%
Due after One Year through Five Years, Amount	$ 7,169
Due after One Year through Five Years, Yield	2.22%
Due after Five Years though Ten Years, Amount	$ 5,483
Due after Five Years though Ten Years, Yield	2.20%
Due after Ten Years, Amount	$ 2,887
Due after Ten Years, Yield	2.33%
Total, Amount	$ 16,263
Total, Yield	2.23%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 724
Due after One Year through Five Years, Amount	7,186
Due after Five Years though Ten Years, Amount	5,608
Due after Ten Years, Amount	2,914
Debt securities carried at fair value	16,432
Debt securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 19,823
Due in One Year or Less, Yield	1.16%
Due after One Year through Five Years, Amount	$ 49,998
Due after One Year through Five Years, Yield	1.84%
Due after Five Years though Ten Years, Amount	$ 36,587
Due after Five Years though Ten Years, Yield	2.49%
Due after Ten Years, Amount	$ 144,641
Due after Ten Years, Yield	3.42%
Total, Amount	$ 251,049
Total, Yield	2.79%
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Due in One Year or Less, Amount	$ 19,833
Due after One Year through Five Years, Amount	50,285
Due after Five Years though Ten Years, Amount	38,071
Due after Ten Years, Amount	146,153
Debt securities carried at fair value	$ 254,342