Investment Securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment		$ 839
Held-to-Maturity Debt Securities, Yield		2.90%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount		$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield		0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount		$ 0
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield		0.00%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount		$ 0
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield		0.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount		$ 839
Held-to-Maturity Securities, Due After Ten Years, Yield		2.90%
Cash Equivalents, at Carrying Value	$ 10	$ 291
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 22,303	$ 18,331
Available-for-sale debt securities, Yield	3.00%	2.80%
Available-for-sale debt securities, Due in one year or less, Amount	$ 216	$ 138
Available-for-sale debt securities, Due in one year or less, Yield	2.30%	2.00%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 1,844	$ 2,046
Available-for-sale debt securities, Due after one year through five years, Yield	2.50%	2.70%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 2,271	$ 2,498
Available-for-sale debt securities, Due after five years through ten years, Yield	2.30%	2.20%
Available-for-sale debt securities, Due after ten years, Amount	$ 17,972	$ 13,649
Available-for-sale debt securities, Due after ten years, Yield	3.10%	2.90%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 22,499	$ 18,651
Amortized cost of available-for-sale debt securities, Due in one year or less, Amount	217	138
Amortized cost of available-for-sale debt securities, Due after one year through five years, Amount	1,852	2,040
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	2,314	2,563
Amortized cost of available-for-sale debt securities, Due after ten years, Amount	18,116	13,910
US Treasury and Government [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 1,777	$ 1,620
Available-for-sale debt securities, Yield	1.70%	1.70%
Available-for-sale debt securities, Due in one year or less, Amount	$ 0	$ 2
Available-for-sale debt securities, Due in one year or less, Yield	0.00%	4.60%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 487	$ 60
Available-for-sale debt securities, Due after one year through five years, Yield	1.70%	1.60%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 1,290	$ 1,558
Available-for-sale debt securities, Due after five years through ten years, Yield	1.80%	1.70%
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	$ 1,831	$ 1,680
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 854	$ 782
Available-for-sale debt securities, Yield	2.90%	3.10%
Available-for-sale debt securities, Due in one year or less, Amount	$ 76	$ 64
Available-for-sale debt securities, Due in one year or less, Yield	1.80%	1.70%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 36	$ 29
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	$ 203	$ 172
Available-for-sale debt securities, Due after five years through ten years, Yield	2.50%	2.80%
Available-for-sale debt securities, Due after ten years, Amount	$ 539	$ 517
Available-for-sale debt securities, Due after ten years, Yield	3.30%	3.40%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 850	$ 794
Foreign Government Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 154	$ 162
Available-for-sale debt securities, Yield	2.50%	2.60%
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	$ 80	$ 58
Available-for-sale debt securities, Due after one year through five years, Yield	2.50%	2.80%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 74	$ 104
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	$ 153	$ 157
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 14,291	$ 10,290
Available-for-sale debt securities, Yield	3.10%	2.90%
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	$ 29
Available-for-sale debt securities, Due after five years through ten years, Yield	2.90%	2.60%
Available-for-sale debt securities, Due after ten years, Amount	$ 14,288	$ 10,261
Available-for-sale debt securities, Due after ten years, Yield	3.10%	2.90%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 14,412	$ 10,473
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 2,494	$ 2,097
Available-for-sale debt securities, Yield	3.10%	2.90%
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	$ 2,494	$ 2,097
Available-for-sale debt securities, Due after ten years, Yield	3.10%	2.90%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 2,517	$ 2,162
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 541	$ 537
Available-for-sale debt securities, Yield	3.20%	2.60%
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	$ 30	$ 0
Available-for-sale debt securities, Due after one year through five years, Yield	3.10%	0.00%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 31	$ 3
Available-for-sale debt securities, Due after five years through ten years, Yield	3.10%	2.80%
Available-for-sale debt securities, Due after ten years, Amount	$ 480	$ 534
Available-for-sale debt securities, Due after ten years, Yield	3.20%	2.60%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 541	$ 537
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 936	$ 1,400
Available-for-sale debt securities, Yield	3.10%	2.80%
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	$ 698	$ 1,059
Available-for-sale debt securities, Due after one year through five years, Yield	3.10%	2.80%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 106	$ 143
Available-for-sale debt securities, Due after five years through ten years, Yield	3.10%	3.20%
Available-for-sale debt securities, Due after ten years, Amount	$ 132	$ 198
Available-for-sale debt securities, Due after ten years, Yield	2.80%	2.60%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 933	$ 1,396
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	$ 1,256	$ 1,443
Available-for-sale debt securities, Yield	2.90%	2.80%
Available-for-sale debt securities, Due in one year or less, Amount	$ 140	$ 72
Available-for-sale debt securities, Due in one year or less, Yield	2.60%	2.20%
Available-for-sale debt securities, Due after one year through five years, Amount	$ 513	$ 840
Available-for-sale debt securities, Due after one year through five years, Yield	2.60%	2.60%
Available-for-sale debt securities, Due after five years through ten years, Amount	$ 564	$ 489
Available-for-sale debt securities, Due after five years through ten years, Yield	3.20%	3.20%
Available-for-sale debt securities, Due after ten years, Amount	$ 39	$ 42
Available-for-sale debt securities, Due after ten years, Yield	4.70%	4.70%
Available-for-sale Debt Securities, Amortized Cost Basis	$ 1,262	$ 1,452
Held-to-maturity Securities [Member]
Schedule of Investments [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 1,899
Held-to-Maturity Debt Securities, Yield	3.10%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield	0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	$ 35
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield	1.70%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 1
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield	3.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,863
Held-to-Maturity Securities, Due After Ten Years, Yield	3.10%
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	$ 1,863
Held-to-Maturity Debt Securities, Yield	3.10%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield	0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	$ 0
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield	0.00%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 0
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield	0.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,863
Held-to-Maturity Securities, Due After Ten Years, Yield	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	$ 36
Held-to-Maturity Debt Securities, Yield	1.70%
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 0
Held-to-Maturity Debt Securities, Due in One Year or Less, Yield	0.00%
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	$ 35
Held-to-Maturity Debt Securities, Due After One Year Through Five Years, Yield	1.70%
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 1
Held-to-Maturity Debt Securities, Due After Five Years Through Ten Years, Yield	3.00%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 0
Held-to-Maturity Securities, Due After Ten Years, Yield	0.00%