Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,647
Due after one year through five years, Amortized Cost	12,150
Due after five years through ten years, Amortized Cost	8,084
Due after ten years, Amortized Cost	2,932
Total debt securities - available-for-sale, Amortized Cost	34,068	$ 32,240
Due in one year or less, Fair Value	3,652
Due after one year through five years, Fair Value	12,298
Due after five years through ten years, Fair Value	8,394
Due after ten years, Fair Value	3,033
Debt Securities, Available-for-sale	34,738	31,898
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	182
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	283
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	136
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	140
Debt Securities, Held-to-maturity	741	621
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	182
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	285
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	136
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	142
Debt Securities, Held-to-maturity, Fair Value	745	620
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	5,662
Total debt securities - available-for-sale, Amortized Cost	5,662	4,947
Mortgage-backed securities, Fair Value	5,730
Debt Securities, Available-for-sale	5,730	4,852
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	1,593
Total debt securities - available-for-sale, Amortized Cost	1,593	1,376
Mortgage-backed securities, Fair Value	1,631
Debt Securities, Available-for-sale	$ 1,631	$ 1,358