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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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