Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,388
Due after one year through five years, Amortized Cost	11,253
Due after five years through ten years, Amortized Cost	9,179
Due after ten years, Amortized Cost	4,326
Total debt securities - available-for-sale, Amortized Cost	36,702	$ 35,328
Due in one year or less, Fair Value	3,407
Due after one year through five years, Fair Value	11,678
Due after five years through ten years, Fair Value	9,826
Due after ten years, Fair Value	4,511
Debt Securities, Available-for-sale	38,337	36,094
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, Year One	400
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year One Through Five	246
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10	30
Debt Securities, Held-to-Maturity, Amortized Cost, Maturity, Allocated and Single Maturity Date, after Year 10	22
Debt Securities, Held-to-maturity	698	972
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	401
Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year One Through Five	252
Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 5 Through 10	31
Debt Securities, Held-to-Maturity, Fair Value, Maturity, Allocated and Single Maturity Date, after Year 10	24
Debt Securities, Held-to-maturity, Fair Value	708	975
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	6,523
Total debt securities - available-for-sale, Amortized Cost	6,523	6,425
Mortgage-backed securities, Fair Value	6,790
Debt Securities, Available-for-sale	6,790	6,528
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	2,033
Total debt securities - available-for-sale, Amortized Cost	2,033	1,811
Mortgage-backed securities, Fair Value	2,125
Debt Securities, Available-for-sale	$ 2,125	$ 1,845