Investment Securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash Equivalents, at Carrying Value	$ 18	$ 10
Held-to-maturity Securities [Member]
Schedule of Investments [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 1,967	$ 1,899
Held-to-Maturity Debt Securities, Yield	3.10%	3.10%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0	$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield	0.00%	0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	$ 30	$ 35
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield	1.70%	1.70%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 1	$ 1
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield	3.00%	3.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,936	$ 1,863
Held-to-Maturity Securities, Due After Ten Years, Yield	3.10%	3.10%
Available-for-sale Securities [Member] | US Treasury and Government [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 1,769	$ 1,777
Available-for-sale debt securities, Yield	1.80%	1.70%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 0
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 481	$ 487
Available-for-sale debt securities, Due after one year through five years, Yield	1.70%	1.70%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 1,288	$ 1,290
Available-for-sale debt securities, Due after five years through ten years, Yield	1.80%	1.80%
Available-for-sale debt securities, Due after ten years, Amount	$ 0	$ 0
Available-for-sale debt securities, Due after ten years, Yield	0.00%	0.00%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 1,855	$ 1,831
Available-for-sale Securities [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 831	$ 854
Available-for-sale debt securities, Yield	3.00%	2.90%
Available-for-sale debt securities, Due in one year or less, Amount	$ 70	$ 76
Available-for-sale debt securities, Due in one year or less, Yield	2.30%	1.80%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 42	$ 36
Available-for-sale debt securities, Due after one year through five years, Yield	2.30%	2.30%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 219	$ 203
Available-for-sale debt securities, Due after five years through ten years, Yield	2.60%	2.50%
Available-for-sale debt securities, Due after ten years, Amount	$ 500	$ 539
Available-for-sale debt securities, Due after ten years, Yield	3.30%	3.30%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 845	$ 850
Available-for-sale Securities [Member] | Foreign Government Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 149	$ 154
Available-for-sale debt securities, Yield	2.50%	2.50%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 0
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 77	$ 80
Available-for-sale debt securities, Due after one year through five years, Yield	2.60%	2.50%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 72	$ 74
Available-for-sale debt securities, Due after five years through ten years, Yield	2.40%	2.40%
Available-for-sale debt securities, Due after ten years, Amount	$ 0	$ 0
Available-for-sale debt securities, Due after ten years, Yield	0.00%	0.00%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 150	$ 153
Available-for-sale Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 14,883	$ 14,291
Available-for-sale debt securities, Yield	3.20%	3.10%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 0
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 0	$ 0
Available-for-sale debt securities, Due after one year through five years, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 3	$ 3
Available-for-sale debt securities, Due after five years through ten years, Yield	2.60%	2.90%
Available-for-sale debt securities, Due after ten years, Amount	$ 14,880	$ 14,288
Available-for-sale debt securities, Due after ten years, Yield	3.20%	3.10%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 15,316	$ 14,412
Available-for-sale Securities [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 2,384	$ 2,494
Available-for-sale debt securities, Yield	3.10%	3.10%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 0
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 0	$ 0
Available-for-sale debt securities, Due after one year through five years, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 0	$ 0
Available-for-sale debt securities, Due after five years through ten years, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after ten years, Amount	$ 2,384	$ 2,494
Available-for-sale debt securities, Due after ten years, Yield	3.10%	3.10%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 2,456	$ 2,517
Available-for-sale Securities [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 580	$ 541
Available-for-sale debt securities, Yield	3.40%	3.20%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 0
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 0	$ 30
Available-for-sale debt securities, Due after one year through five years, Yield	0.00%	3.10%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 31	$ 31
Available-for-sale debt securities, Due after five years through ten years, Yield	3.30%	3.10%
Available-for-sale debt securities, Due after ten years, Amount	$ 549	$ 480
Available-for-sale debt securities, Due after ten years, Yield	3.40%	3.20%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 582	$ 541
Available-for-sale Securities [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 900	$ 936
Available-for-sale debt securities, Yield	3.20%	3.10%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 0
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	0.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 655	$ 698
Available-for-sale debt securities, Due after one year through five years, Yield	3.20%	3.10%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 121	$ 106
Available-for-sale debt securities, Due after five years through ten years, Yield	3.30%	3.10%
Available-for-sale debt securities, Due after ten years, Amount	$ 124	$ 132
Available-for-sale debt securities, Due after ten years, Yield	2.90%	2.80%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 901	$ 933
Available-for-sale Securities [Member] | Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 1,230	$ 1,256
Available-for-sale debt securities, Yield	3.00%	2.90%
Available-for-sale debt securities, Due in one year or less, Amount	$ 123	$ 140
Available-for-sale debt securities, Due in one year or less, Yield	2.80%	2.60%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 496	$ 513
Available-for-sale debt securities, Due after one year through five years, Yield	2.70%	2.60%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 573	$ 564
Available-for-sale debt securities, Due after five years through ten years, Yield	3.30%	3.20%
Available-for-sale debt securities, Due after ten years, Amount	$ 38	$ 39
Available-for-sale debt securities, Due after ten years, Yield	4.80%	4.70%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 1,263	$ 1,262
Available-for-sale Securities [Member] | Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 22,726	$ 22,303
Available-for-sale debt securities, Yield	3.00%	3.00%
Available-for-sale debt securities, Due in one year or less, Amount	$ 193	$ 216
Available-for-sale debt securities, Due in one year or less, Yield	2.60%	2.30%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 1,751	$ 1,844
Available-for-sale debt securities, Due after one year through five years, Yield	2.60%	2.50%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 2,307	$ 2,271
Available-for-sale debt securities, Due after five years through ten years, Yield	2.30%	2.30%
Available-for-sale debt securities, Due after ten years, Amount	$ 18,475	$ 17,972
Available-for-sale debt securities, Due after ten years, Yield	3.20%	3.10%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 23,368	$ 22,499
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	194	217
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	1,776	1,852
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	2,403	2,314
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	18,995	18,116
Held-to-maturity Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Investments [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 1,936	$ 1,863
Held-to-Maturity Debt Securities, Yield	3.10%	3.10%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0	$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield	0.00%	0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	$ 0	$ 0
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield	0.00%	0.00%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 0	$ 0
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield	0.00%	0.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,936	$ 1,863
Held-to-Maturity Securities, Due After Ten Years, Yield	3.10%	3.10%
Held-to-maturity Securities [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 31	$ 36
Held-to-Maturity Debt Securities, Yield	1.70%	1.70%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0	$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield	0.00%	0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	$ 30	$ 35
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield	1.70%	1.70%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 1	$ 1
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield	3.00%	3.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 0	$ 0
Held-to-Maturity Securities, Due After Ten Years, Yield	0.00%	0.00%