Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,457
Due after one year through five years, Amortized Cost	12,283
Due after five years through ten years, Amortized Cost	7,314
Due after ten years, Amortized Cost	2,711
Total debt securities - available-for-sale, Amortized Cost	32,448	$ 32,240
Due in one year or less, Fair Value	3,455
Due after one year through five years, Fair Value	12,304
Due after five years through ten years, Fair Value	7,430
Due after ten years, Fair Value	2,765
Debt Securities, Available-for-sale	32,630	31,898
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	132
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	318
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	131
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	143
Held-to-maturity securities, Amortized Cost	724	621
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	132
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	318
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	131
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	144
Debt Securities, Held-to-maturity, Fair Value	725	620
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	5,212
Total debt securities - available-for-sale, Amortized Cost	5,212	4,947
Mortgage-backed securities, Fair Value	5,198
Debt Securities, Available-for-sale	5,198	4,852
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	1,471
Total debt securities - available-for-sale, Amortized Cost	1,471	1,376
Mortgage-backed securities, Fair Value	1,478
Debt Securities, Available-for-sale	$ 1,478	$ 1,358