Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,010
Due after one year through five years, Amortized Cost	11,565
Due after five years through ten years, Amortized Cost	8,563
Due after ten years, Amortized Cost	3,655
Total debt securities - available-for-sale, Amortized Cost	35,233	$ 35,328
Due in one year or less, Fair Value	3,011
Due after one year through five years, Fair Value	11,604
Due after five years through ten years, Fair Value	8,669
Due after ten years, Fair Value	3,656
Debt Securities, Available-for-sale	35,632	36,094
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One	319
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five	352
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10	28
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10	26
Debt Securities, Held-to-maturity	725	972
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	320
Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five	359
Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10	28
Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10	28
Debt Securities, Held-to-maturity, Fair Value	735	975
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	6,508
Total debt securities - available-for-sale, Amortized Cost	6,508	6,425
Mortgage-backed securities, Fair Value	6,768
Debt Securities, Available-for-sale	6,768	6,528
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	1,932
Total debt securities - available-for-sale, Amortized Cost	1,932	1,811
Mortgage-backed securities, Fair Value	1,924
Debt Securities, Available-for-sale	$ 1,924	$ 1,845