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Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 41,749	$ 38,079
Securities, Available for sale Debt Securities, Gross Unrealized Gains	1,177	1,771
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(169)	(22)
Debt Securities, Available-for-sale	42,757	39,828
Held-to-maturity securities, Amortized Cost	616	638
Securities, Held to maturity, Unrecognized Holding Gain	5	9
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1)	0
Debt Securities, Held-to-maturity, Fair Value	620	647
Total debt securities, Amortized Cost	42,365	38,717
Total debt securities, Gross Unrealized Gains	1,182	1,780
Total debt securities, Gross Unrealized Losses	(170)	(22)
Debt securities, Fair Value Disclosure	43,377	40,475
Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale	42,757	39,828
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,625	3,335
Securities, Available for sale Debt Securities, Gross Unrealized Gains	70	133
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(23)	(3)
Debt Securities, Available-for-sale	3,672	3,465
Held-to-maturity securities, Amortized Cost	506	420
Securities, Held to maturity, Unrecognized Holding Gain	3	6
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1)	0
Debt Securities, Held-to-maturity, Fair Value	508	426
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	7,093	6,893
Securities, Available for sale Debt Securities, Gross Unrealized Gains	341	435
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(15)	0
Debt Securities, Available-for-sale	7,419	7,328
Held-to-maturity securities, Amortized Cost	30	31
Securities, Held to maturity, Unrecognized Holding Gain	2	2
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	32	33
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	21,987	18,886
Securities, Available for sale Debt Securities, Gross Unrealized Gains	547	863
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(84)	(12)
Debt Securities, Available-for-sale	22,450	19,737
Held-to-maturity securities, Amortized Cost	80	187
Securities, Held to maturity, Unrecognized Holding Gain	0	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	80	188
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	6,281	6,849
Securities, Available for sale Debt Securities, Gross Unrealized Gains	156	245
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(34)	(3)
Debt Securities, Available-for-sale	6,403	7,091
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,763	2,116
Securities, Available for sale Debt Securities, Gross Unrealized Gains	63	95
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(13)	(4)
Debt Securities, Available-for-sale	$ 2,813	$ 2,207

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