Investment Securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 29,181	$ 30,284
Cash equivalents	12	73
Held-to-maturity securities
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost, before other-than-temporary impairment	$ 1,497	$ 1,568
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	$ 17	$ 21
Debt securities, held-to-maturity, due after one year through five years, yield	2.30%	2.20%
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	$ 1,480	$ 1,547
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	$ 806	$ 2,048
Debt securities, available-for-sale, yield	1.20%	1.50%
Debt securities, available-for-sale, due in one year or less	$ 14	$ 65
Debt securities, available-for-sale, due in one year or less, yield	0.30%	2.10%
Debt securities, available-for-sale, due after one year through five years	$ 708	$ 1,590
Debt securities, available-for-sale, due after one year through five years, yield	1.20%	1.40%
Debt securities, available-for-sale, due after five years through ten years	$ 84	$ 393
Debt securities, available-for-sale, due after five years through ten years, yield	1.70%	1.70%
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	$ 785	$ 2,059
Available-for-sale securities | US States and political subdivisions
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 709	$ 641
Debt securities, available-for-sale, yield	3.10%	3.10%
Debt securities, available-for-sale, due in one year or less	$ 25	$ 22
Debt securities, available-for-sale, due in one year or less, yield	1.90%	2.70%
Debt securities, available-for-sale, due after one year through five years	$ 75	$ 75
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	$ 160	$ 159
Debt securities, available-for-sale, due after five years through ten years, yield	2.90%	2.80%
Debt securities, available-for-sale, due after ten years	$ 449	$ 385
Debt securities, available-for-sale, due after ten years, yield	3.40%	3.40%
Debt securities, available-for-sale, amortized cost	$ 691	$ 623
Available-for-sale securities | Foreign government
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 180	$ 186
Debt securities, available-for-sale, yield	1.90%	1.90%
Debt securities, available-for-sale, due in one year or less	$ 35	$ 35
Debt securities, available-for-sale, due in one year or less, yield	0.50%	0.40%
Debt securities, available-for-sale, due after one year through five years	$ 63	$ 65
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	$ 82	$ 86
Debt securities, available-for-sale, due after five years through ten years, yield	2.30%	2.30%
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	$ 175	$ 184
Available-for-sale securities | Agency mortgage-backed residential
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 21,533	$ 21,404
Debt securities, available-for-sale, yield	3.20%	3.20%
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	$ 1	$ 0
Debt securities, available-for-sale, due after one year through five years, yield	2.90%	0.00%
Debt securities, available-for-sale, due after five years through ten years	$ 47	$ 47
Debt securities, available-for-sale, due after five years through ten years, yield	2.00%	2.00%
Debt securities, available-for-sale, due after ten years	$ 21,485	$ 21,357
Debt securities, available-for-sale, due after ten years, yield	3.20%	3.20%
Debt securities, available-for-sale, amortized cost	$ 20,702	$ 21,183
Available-for-sale securities | Mortgage-backed residential
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 2,948	$ 2,850
Debt securities, available-for-sale, yield	3.30%	3.20%
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	$ 2,948	$ 2,850
Debt securities, available-for-sale, due after ten years, yield	3.30%	3.20%
Debt securities, available-for-sale, amortized cost	$ 2,968	$ 2,841
Available-for-sale securities | Agency mortgage-backed commercial
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 1,447	$ 1,382
Debt securities, available-for-sale, yield	2.80%	2.90%
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%	3.20%
Debt securities, available-for-sale, due after five years through ten years	$ 1,175	$ 1,109
Debt securities, available-for-sale, due after five years through ten years, yield	2.90%	3.00%
Debt securities, available-for-sale, due after ten years	$ 269	$ 270
Debt securities, available-for-sale, due after ten years, yield	2.50%	2.40%
Debt securities, available-for-sale, amortized cost	$ 1,333	$ 1,344
Available-for-sale securities | Mortgage-backed commercial
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 37	$ 42
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	$ 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	$ 37	$ 42
Debt securities, available-for-sale, due after ten years, yield	3.50%	3.50%
Debt securities, available-for-sale, amortized cost	$ 41	$ 41
Available-for-sale securities | Asset-backed
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 329	$ 368
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	$ 275	$ 317
Debt securities, available-for-sale, due after one year through five years, yield	3.60%	3.60%
Debt securities, available-for-sale, due after five years through ten years	$ 14	$ 5
Debt securities, available-for-sale, due after five years through ten years, yield	2.90%	2.70%
Debt securities, available-for-sale, due after ten years	$ 40	$ 46
Debt securities, available-for-sale, due after ten years, yield	3.00%	3.00%
Debt securities, available-for-sale, amortized cost	$ 333	$ 365
Available-for-sale securities | Corporate debt
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 1,192	$ 1,363
Debt securities, available-for-sale, yield	3.10%	3.20%
Debt securities, available-for-sale, due in one year or less	$ 135	$ 125
Debt securities, available-for-sale, due in one year or less, yield	2.80%	2.90%
Debt securities, available-for-sale, due after one year through five years	$ 520	$ 580
Debt securities, available-for-sale, due after one year through five years, yield	3.00%	3.00%
Debt securities, available-for-sale, due after five years through ten years	$ 525	$ 649
Debt securities, available-for-sale, due after five years through ten years, yield	3.30%	3.40%
Debt securities, available-for-sale, due after ten years	$ 12	$ 9
Debt securities, available-for-sale, due after ten years, yield	3.00%	3.30%
Debt securities, available-for-sale, amortized cost	$ 1,203	$ 1,327
Available-for-sale securities | Available-for-sale debt securities
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 29,181	$ 30,284
Debt securities, available-for-sale, yield	3.10%	3.10%
Debt securities, available-for-sale, due in one year or less	$ 209	$ 247
Debt securities, available-for-sale, due in one year or less, yield	2.20%	2.30%
Debt securities, available-for-sale, due after one year through five years	$ 1,645	$ 2,630
Debt securities, available-for-sale, due after one year through five years, yield	2.20%	2.10%
Debt securities, available-for-sale, due after five years through ten years	$ 2,087	$ 2,448
Debt securities, available-for-sale, due after five years through ten years, yield	2.90%	2.80%
Debt securities, available-for-sale, due after ten years	$ 25,240	$ 24,959
Debt securities, available-for-sale, due after ten years, yield	3.20%	3.20%
Debt securities, available-for-sale, amortized cost	$ 28,231	$ 29,967
Debt securities, available-for-sale, maturity, within one year, amortized cost	209	246
Debt securities, available-for-sale, maturity, after one through five years, amortized cost	1,636	2,624
Debt securities, available-for-sale, maturity, after five through ten years, amortized cost	1,972	2,378
Debt securities, available-for-sale, maturity, after ten years, amortized cost	24,414	24,719
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	$ 1,480	$ 1,547
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	$ 1,480	$ 1,547
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	$ 17	$ 21
Debt securities, held-to-maturity, yield	2.30%	2.20%
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	$ 17	$ 21
Debt securities, held-to-maturity, due after one year through five years, yield	2.30%	2.20%
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	$ 0	$ 0
Debt securities, held-to-maturity, due after ten years, yield	0.00%	0.00%