Investment Securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 29,384	$ 25,303
Cash equivalents	124	35
Held-to-maturity securities
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost, before other-than-temporary impairment	$ 2,618	$ 2,362
Debt securities, held-to-maturity, yield	3.20%	3.20%
Debt securities, held-to-maturity, maturity, within one year, amortized Cost	$ 0	$ 0
Debt securities, held-to-maturity, due in one year or less, yield	0.00%	0.00%
Debt securities, held-to-maturity, maturity, one through five years, amortized Cost	$ 25	$ 42
Debt securities, held-to-maturity, due after one year through five years, yield	2.20%	2.00%
Debt securities, held-to-maturity, maturity, after five through ten years, amortized cost	$ 0	$ 1
Debt securities, held-to-maturity, due after five years through ten years, yield	0.00%	3.30%
Debt securities, held-to-maturity, maturity, after 10 Years, amortized cost	$ 2,593	$ 2,319
Debt securities, held-to-maturity, due after ten years, yield	3.20%	3.20%
Available-for-sale securities | US Treasury
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 2,382	$ 1,851
Debt securities, available-for-sale, yield	1.50%	1.90%
Debt securities, available-for-sale, due in one year or less	$ 65	$ 12
Debt securities, available-for-sale, due in one year or less, yield	2.10%	1.00%
Debt securities, available-for-sale, due after one year through five years	$ 1,742	$ 1,277
Debt securities, available-for-sale, due after one year through five years, yield	1.50%	1.80%
Debt securities, available-for-sale, due after five years through ten years	$ 575	$ 562
Debt securities, available-for-sale, due after five years through ten years, yield	1.60%	2.00%
Debt securities, available-for-sale, due after ten years	$ 0	$ 0
Debt securities, available-for-sale, due after ten years, yield	0.00%	0.00%
Debt securities, available-for-sale, amortized cost	$ 2,389	$ 1,911
Available-for-sale securities | US States and political subdivisions
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 630	$ 802
Debt securities, available-for-sale, yield	3.10%	3.00%
Debt securities, available-for-sale, due in one year or less	$ 21	$ 49
Debt securities, available-for-sale, due in one year or less, yield	2.20%	1.90%
Debt securities, available-for-sale, due after one year through five years	$ 61	$ 43
Debt securities, available-for-sale, due after one year through five years, yield	2.20%	2.30%
Debt securities, available-for-sale, due after five years through ten years	$ 169	$ 252
Debt securities, available-for-sale, due after five years through ten years, yield	2.80%	2.60%
Debt securities, available-for-sale, due after ten years	$ 379	$ 458
Debt securities, available-for-sale, due after ten years, yield	3.40%	3.40%
Debt securities, available-for-sale, amortized cost	$ 612	$ 816
Available-for-sale securities | Foreign government
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 155	$ 145
Debt securities, available-for-sale, yield	2.30%	2.40%
Debt securities, available-for-sale, due in one year or less	$ 4	$ 18
Debt securities, available-for-sale, due in one year or less, yield	1.40%	3.10%
Debt securities, available-for-sale, due after one year through five years	$ 65	$ 60
Debt securities, available-for-sale, due after one year through five years, yield	2.30%	2.30%
Debt securities, available-for-sale, due after five years through ten years	$ 86	$ 67
Debt securities, available-for-sale, due after five years through ten years, yield	2.30%	2.40%
Debt securities, available-for-sale, due after ten years	$ 0	$ 0
Debt securities, available-for-sale, due after ten years, yield	0.00%	0.00%
Debt securities, available-for-sale, amortized cost	$ 152	$ 145
Available-for-sale securities | Agency mortgage-backed residential
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 20,108	$ 17,138
Debt securities, available-for-sale, yield	3.30%	3.30%
Debt securities, available-for-sale, due in one year or less	$ 0	$ 0
Debt securities, available-for-sale, due in one year or less, yield	0.00%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 0	$ 0
Debt securities, available-for-sale, due after one year through five years, yield	0.00%	0.00%
Debt securities, available-for-sale, due after five years through ten years	$ 49	$ 54
Debt securities, available-for-sale, due after five years through ten years, yield	2.00%	1.90%
Debt securities, available-for-sale, due after ten years	$ 20,059	$ 17,084
Debt securities, available-for-sale, due after ten years, yield	3.30%	3.30%
Debt securities, available-for-sale, amortized cost	$ 19,887	$ 17,486
Available-for-sale securities | Mortgage-backed residential
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 2,812	$ 2,686
Debt securities, available-for-sale, yield	3.30%	3.30%
Debt securities, available-for-sale, due in one year or less	$ 0	$ 0
Debt securities, available-for-sale, due in one year or less, yield	0.00%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 0	$ 0
Debt securities, available-for-sale, due after one year through five years, yield	0.00%	0.00%
Debt securities, available-for-sale, due after five years through ten years	$ 0	$ 0
Debt securities, available-for-sale, due after five years through ten years, yield	0.00%	0.00%
Debt securities, available-for-sale, due after ten years	$ 2,812	$ 2,686
Debt securities, available-for-sale, due after ten years, yield	3.30%	3.30%
Debt securities, available-for-sale, amortized cost	$ 2,799	$ 2,796
Available-for-sale securities | Agency mortgage-backed commercial
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 1,413	$ 3
Debt securities, available-for-sale, yield	2.90%	3.10%
Debt securities, available-for-sale, due in one year or less	$ 0	$ 0
Debt securities, available-for-sale, due in one year or less, yield	0.00%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 3	$ 3
Debt securities, available-for-sale, due after one year through five years, yield	3.20%	3.10%
Debt securities, available-for-sale, due after five years through ten years	$ 1,089	$ 0
Debt securities, available-for-sale, due after five years through ten years, yield	3.00%	0.00%
Debt securities, available-for-sale, due after ten years	$ 321	$ 0
Debt securities, available-for-sale, due after ten years, yield	2.60%	0.00%
Debt securities, available-for-sale, amortized cost	$ 1,341	$ 3
Available-for-sale securities | Mortgage-backed commercial
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 113	$ 714
Debt securities, available-for-sale, yield	3.50%	3.80%
Debt securities, available-for-sale, due in one year or less	$ 0	$ 0
Debt securities, available-for-sale, due in one year or less, yield	0.00%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 0	$ 0
Debt securities, available-for-sale, due after one year through five years, yield	0.00%	0.00%
Debt securities, available-for-sale, due after five years through ten years	$ 0	$ 46
Debt securities, available-for-sale, due after five years through ten years, yield	0.00%	3.90%
Debt securities, available-for-sale, due after ten years	$ 113	$ 668
Debt securities, available-for-sale, due after ten years, yield	3.50%	3.80%
Debt securities, available-for-sale, amortized cost	$ 112	$ 715
Available-for-sale securities | Asset-backed
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 417	$ 723
Debt securities, available-for-sale, yield	3.50%	3.50%
Debt securities, available-for-sale, due in one year or less	$ 0	$ 0
Debt securities, available-for-sale, due in one year or less, yield	0.00%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 310	$ 478
Debt securities, available-for-sale, due after one year through five years, yield	3.50%	3.40%
Debt securities, available-for-sale, due after five years through ten years	$ 53	$ 121
Debt securities, available-for-sale, due after five years through ten years, yield	3.90%	4.00%
Debt securities, available-for-sale, due after ten years	$ 54	$ 124
Debt securities, available-for-sale, due after ten years, yield	3.00%	3.30%
Debt securities, available-for-sale, amortized cost	$ 413	$ 723
Available-for-sale securities | Corporate debt
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 1,354	$ 1,241
Debt securities, available-for-sale, yield	3.20%	3.10%
Debt securities, available-for-sale, due in one year or less	$ 125	$ 144
Debt securities, available-for-sale, due in one year or less, yield	2.80%	2.80%
Debt securities, available-for-sale, due after one year through five years	$ 571	$ 496
Debt securities, available-for-sale, due after one year through five years, yield	3.00%	2.90%
Debt securities, available-for-sale, due after five years through ten years	$ 643	$ 581
Debt securities, available-for-sale, due after five years through ten years, yield	3.40%	3.30%
Debt securities, available-for-sale, due after ten years	$ 15	$ 20
Debt securities, available-for-sale, due after ten years, yield	5.50%	5.50%
Debt securities, available-for-sale, amortized cost	$ 1,321	$ 1,286
Available-for-sale securities | Available-for-sale debt securities
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 29,384	$ 25,303
Debt securities, available-for-sale, yield	3.10%	3.20%
Debt securities, available-for-sale, due in one year or less	$ 215	$ 223
Debt securities, available-for-sale, due in one year or less, yield	2.50%	2.60%
Debt securities, available-for-sale, due after one year through five years	$ 2,752	$ 2,357
Debt securities, available-for-sale, due after one year through five years, yield	2.00%	2.40%
Debt securities, available-for-sale, due after five years through ten years	$ 2,664	$ 1,683
Debt securities, available-for-sale, due after five years through ten years, yield	2.80%	2.80%
Debt securities, available-for-sale, due after ten years	$ 23,753	$ 21,040
Debt securities, available-for-sale, due after ten years, yield	3.30%	3.30%
Debt securities, available-for-sale, amortized cost	$ 29,026	$ 25,881
Debt securities, available-for-sale, maturity, within one year, amortized cost	215	224
Debt securities, available-for-sale, maturity, after one through five years, amortized cost	2,745	2,405
Debt securities, available-for-sale, maturity, after five through ten years, amortized cost	2,568	1,743
Debt securities, available-for-sale, maturity, after 10 Years, amortized cost	23,498	21,509
Held-to-maturity securities | Agency mortgage-backed residential
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost, before other-than-temporary impairment	$ 2,593	$ 2,319
Debt securities, held-to-maturity, yield	3.20%	3.20%
Debt securities, held-to-maturity, maturity, within one year, amortized Cost	$ 0	$ 0
Debt securities, held-to-maturity, due in one year or less, yield	0.00%	0.00%
Debt securities, held-to-maturity, maturity, one through five years, amortized Cost	$ 0	$ 0
Debt securities, held-to-maturity, due after one year through five years, yield	0.00%	0.00%
Debt securities, held-to-maturity, maturity, after five through ten years, amortized cost	$ 0	$ 0
Debt securities, held-to-maturity, due after five years through ten years, yield	0.00%	0.00%
Debt securities, held-to-maturity, maturity, after 10 Years, amortized cost	$ 2,593	$ 2,319
Debt securities, held-to-maturity, due after ten years, yield	3.20%	3.20%
Held-to-maturity securities | Asset-backed
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost, before other-than-temporary impairment	$ 25	$ 43
Debt securities, held-to-maturity, yield	2.20%	2.00%
Debt securities, held-to-maturity, maturity, within one year, amortized Cost	$ 0	$ 0
Debt securities, held-to-maturity, due in one year or less, yield	0.00%	0.00%
Debt securities, held-to-maturity, maturity, one through five years, amortized Cost	$ 25	$ 42
Debt securities, held-to-maturity, due after one year through five years, yield	2.20%	2.00%
Debt securities, held-to-maturity, maturity, after five through ten years, amortized cost	$ 0	$ 1
Debt securities, held-to-maturity, due after five years through ten years, yield	0.00%	3.30%
Debt securities, held-to-maturity, maturity, after 10 Years, amortized cost	$ 0	$ 0
Debt securities, held-to-maturity, due after ten years, yield	0.00%	0.00%