Investment Securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash equivalents	$ 54	$ 10
Held-to-maturity securities
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost, before other-than-temporary impairment	$ 2,246	$ 1,899
Debt securities, held-to-maturity, yield	3.10%	3.10%
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, after one through five years, amortized cost	$ 48	$ 35
Debt securities, held-to-maturity, due after one year through five years, yield	2.00%	1.70%
Debt securities, held-to-maturity, maturity, after five through ten years, amortized cost	$ 1	$ 1
Debt securities, held-to-maturity, due after five years through ten years, yield	3.30%	3.00%
Debt securities, held-to-maturity, maturity, after 10 Years, amortized cost	$ 2,197	$ 1,863
Debt securities, held-to-maturity, due after ten years, yield	3.20%	3.10%
Available-for-sale securities | US Treasury
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 1,904	$ 1,777
Debt securities, available-for-sale, yield	1.90%	1.70%
Debt securities, available-for-sale, due in one year or less	$ 7	$ 0
Debt securities, available-for-sale, due in one year or less, yield	1.70%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 932	$ 487
Debt securities, available-for-sale, due after one year through five years, yield	1.90%	1.70%
Debt securities, available-for-sale, due after five years through ten years	$ 965	$ 1,290
Debt securities, available-for-sale, due after five years through ten years, yield	1.90%	1.80%
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	$ 2,007	$ 1,831
Available-for-sale securities | US States and political subdivisions
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 865	$ 854
Debt securities, available-for-sale, yield	3.10%	2.90%
Debt securities, available-for-sale, due in one year or less	$ 44	$ 76
Debt securities, available-for-sale, due in one year or less, yield	2.50%	1.80%
Debt securities, available-for-sale, due after one year through five years	$ 57	$ 36
Debt securities, available-for-sale, due after one year through five years, yield	2.40%	2.30%
Debt securities, available-for-sale, due after five years through ten years	$ 257	$ 203
Debt securities, available-for-sale, due after five years through ten years, yield	2.60%	2.50%
Debt securities, available-for-sale, due after ten years	$ 507	$ 539
Debt securities, available-for-sale, due after ten years, yield	3.50%	3.30%
Debt securities, available-for-sale, amortized cost	$ 887	$ 850
Available-for-sale securities | Foreign government
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 155	$ 154
Debt securities, available-for-sale, yield	2.50%	2.50%
Debt securities, available-for-sale, due in one year or less	$ 18	$ 0
Debt securities, available-for-sale, due in one year or less, yield	3.40%	0.00%
Debt securities, available-for-sale, due after one year through five years	$ 61	$ 80
Debt securities, available-for-sale, due after one year through five years, yield	2.30%	2.50%
Debt securities, available-for-sale, due after five years through ten years	$ 73	$ 74
Debt securities, available-for-sale, due after five years through ten years, yield	2.40%	2.40%
Debt securities, available-for-sale, due after ten years	$ 3	$ 0
Debt securities, available-for-sale, due after ten years, yield	2.90%	0.00%
Debt securities, available-for-sale, amortized cost	$ 158	$ 153
Available-for-sale securities | Agency mortgage-backed residential
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 16,014	$ 14,291
Debt securities, available-for-sale, yield	3.20%	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	$ 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	$ 56	$ 3
Debt securities, available-for-sale, due after five years through ten years, yield	1.90%	2.90%
Debt securities, available-for-sale, due after ten years	$ 15,958	$ 14,288
Debt securities, available-for-sale, due after ten years, yield	3.20%	3.10%
Debt securities, available-for-sale, amortized cost	$ 16,641	$ 14,412
Available-for-sale securities | Mortgage-backed residential
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 2,561	$ 2,494
Debt securities, available-for-sale, yield	3.20%	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	$ 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	$ 2,561	$ 2,494
Debt securities, available-for-sale, due after ten years, yield	3.20%	3.10%
Debt securities, available-for-sale, amortized cost	$ 2,670	$ 2,517
Available-for-sale securities | Mortgage-backed commercial
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 631	$ 541
Debt securities, available-for-sale, yield	3.60%	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	$ 3	$ 30
Debt securities, available-for-sale, due after one year through five years, yield	3.00%	3.10%
Debt securities, available-for-sale, due after five years through ten years	$ 46	$ 31
Debt securities, available-for-sale, due after five years through ten years, yield	3.60%	3.10%
Debt securities, available-for-sale, due after ten years	$ 582	$ 480
Debt securities, available-for-sale, due after ten years, yield	3.60%	3.20%
Debt securities, available-for-sale, amortized cost	$ 632	$ 541
Available-for-sale securities | Asset-backed
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 733	$ 936
Debt securities, available-for-sale, yield	3.30%	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	$ 533	$ 698
Debt securities, available-for-sale, due after one year through five years, yield	3.30%	3.10%
Debt securities, available-for-sale, due after five years through ten years	$ 99	$ 106
Debt securities, available-for-sale, due after five years through ten years, yield	3.70%	3.10%
Debt securities, available-for-sale, due after ten years	$ 101	$ 132
Debt securities, available-for-sale, due after ten years, yield	3.00%	2.80%
Debt securities, available-for-sale, amortized cost	$ 735	$ 933
Available-for-sale securities | Corporate debt
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 1,259	$ 1,256
Debt securities, available-for-sale, yield	3.10%	2.90%
Debt securities, available-for-sale, due in one year or less	$ 134	$ 140
Debt securities, available-for-sale, due in one year or less, yield	2.90%	2.60%
Debt securities, available-for-sale, due after one year through five years	$ 515	$ 513
Debt securities, available-for-sale, due after one year through five years, yield	2.80%	2.60%
Debt securities, available-for-sale, due after five years through ten years	$ 582	$ 564
Debt securities, available-for-sale, due after five years through ten years, yield	3.30%	3.20%
Debt securities, available-for-sale, due after ten years	$ 28	$ 39
Debt securities, available-for-sale, due after ten years, yield	5.10%	4.70%
Debt securities, available-for-sale, amortized cost	$ 1,302	$ 1,262
Available-for-sale securities | Available-for-sale debt securities
Schedule of Investments [Line Items]
Total available-for-sale debt securities	$ 24,122	$ 22,303
Debt securities, available-for-sale, yield	3.10%	3.00%
Debt securities, available-for-sale, due in one year or less	$ 203	$ 216
Debt securities, available-for-sale, due in one year or less, yield	2.80%	2.30%
Debt securities, available-for-sale, due after one year through five years	$ 2,101	$ 1,844
Debt securities, available-for-sale, due after one year through five years, yield	2.50%	2.50%
Debt securities, available-for-sale, due after five years through ten years	$ 2,078	$ 2,271
Debt securities, available-for-sale, due after five years through ten years, yield	2.50%	2.30%
Debt securities, available-for-sale, due after ten years	$ 19,740	$ 17,972
Debt securities, available-for-sale, due after ten years, yield	3.30%	3.10%
Debt securities, available-for-sale, amortized cost	$ 25,032	$ 22,499
Debt securities, available-for-sale, maturity, within one year, amortized cost	203	217
Debt securities, available-for-sale, maturity, after one through five years, amortized cost	2,154	1,852
Debt securities, available-for-sale, maturity, after five through ten years, amortized cost	2,181	2,314
Debt securities, available-for-sale, maturity, after 10 Years, amortized cost	20,494	18,116
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	$ 2,197	$ 1,863
Debt securities, held-to-maturity, yield	3.20%	3.10%
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, after 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	$ 2,197	$ 1,863
Debt securities, held-to-maturity, due after ten years, yield	3.20%	3.10%
Held-to-maturity securities | Asset-backed
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost, before other-than-temporary impairment	$ 49	$ 36
Debt securities, held-to-maturity, yield	2.00%	1.70%
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, after one through five years, amortized cost	$ 48	$ 35
Debt securities, held-to-maturity, due after one year through five years, yield	2.00%	1.70%
Debt securities, held-to-maturity, maturity, after five through ten years, amortized cost	$ 1	$ 1
Debt securities, held-to-maturity, due after five years through ten years, yield	3.30%	3.00%
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%