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Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | $ 38,079 | $ 35,328
Securities, Available for sale Debt Securities, Gross Unrealized Gains | 1,771 | 795
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (22) | (29)
Debt Securities, Available-for-sale | 39,828 | 36,094
Held-to-maturity securities, Amortized Cost | 638 | 972
Securities, Held to maturity, Unrecognized Holding Gain | 9 | 4
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | (1)
Debt Securities, Held-to-maturity, Fair Value | 647 | 975
Total investments, Amortized Cost | 38,717 | 36,300
Total investments, Gross Unrealized Gains | 1,780 | 799
Total investments, Gross Unrealized Losses | (22) | (30)
Investments, Fair Value Disclosure | 40,475 | 37,069
Debt Securities - available-for-sale |  |
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale | 39,828 | 36,094
U.S. Government and Agency Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 3,335 | 3,502
Securities, Available for sale Debt Securities, Gross Unrealized Gains | 133 | 55
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (3) | (4)
Debt Securities, Available-for-sale | 3,465 | 3,553
Held-to-maturity securities, Amortized Cost | 420 | 402
Securities, Held to maturity, Unrecognized Holding Gain | 6 | 2
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | 0
Debt Securities, Held-to-maturity, Fair Value | 426 | 404
State and Municipal Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 6,893 | 5,680
Securities, Available for sale Debt Securities, Gross Unrealized Gains | 435 | 251
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | (5)
Debt Securities, Available-for-sale | 7,328 | 5,926
Held-to-maturity securities, Amortized Cost | 31 | 32
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 | 33 | 34
Corporate Obligations [Member] |  |
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 18,886 | 17,910
Securities, Available for sale Debt Securities, Gross Unrealized Gains | 863 | 343
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (12) | (11)
Debt Securities, Available-for-sale | 19,737 | 18,242
Held-to-maturity securities, Amortized Cost | 187 | 538
Securities, Held to maturity, Unrecognized Holding Gain | 1 | 0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss | 0 | (1)
Debt Securities, Held-to-maturity, Fair Value | 188 | 537
U.S. Agency Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 6,849 | 6,425
Securities, Available for sale Debt Securities, Gross Unrealized Gains | 245 | 109
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (3) | (6)
Debt Securities, Available-for-sale | 7,091 | 6,528
Non-U.S. Agency Mortgage-Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,116 | 1,811
Securities, Available for sale Debt Securities, Gross Unrealized Gains | 95 | 37
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (4) | (3)
Debt Securities, Available-for-sale | $ 2,207 | $ 1,845

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