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Fair Value Measurements - Schedule of Fair Value Measurements by Level for Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Assets    
Cash and cash equivalents $ 21,264 $ 17,888
Investments:    
Equity securities 1 1
Total assets at fair value 40,666 37,635
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 532 535
Corporate securities    
Investments:    
Fair value 10,592 10,662
Municipal securities    
Investments:    
Fair value 3,689 3,758
Short-term time deposits    
Investments:    
Fair value 68 205
Asset-backed securities    
Investments:    
Fair value 1,612 1,666
Residential mortgage-backed securities    
Investments:    
Fair value 1,703 1,714
Commercial mortgage-backed securities    
Investments:    
Fair value 1,115 1,136
Level I    
Assets    
Cash and cash equivalents 21,264 17,888
Investments:    
Total assets at fair value 21,885 18,492
Level II    
Assets    
Cash and cash equivalents 0 0
Investments:    
Total assets at fair value 18,781 19,143
Level III    
Assets    
Cash and cash equivalents 0 0
Investments:    
Total assets at fair value 0 0
Investments:    
Investments:    
Fair value 17,969 18,334
Total investments 17,970 18,335
Investments: | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 53 55
Investments: | Corporate securities    
Investments:    
Fair value 10,558 10,652
Investments: | Municipal securities    
Investments:    
Fair value 2,860 2,906
Investments: | Short-term time deposits    
Investments:    
Fair value 68 205
Investments: | Asset-backed securities    
Investments:    
Fair value 1,612 1,666
Investments: | Residential mortgage-backed securities    
Investments:    
Fair value 1,703 1,714
Investments: | Commercial mortgage-backed securities    
Investments:    
Fair value 1,115 1,136
Investments: | Equity securities    
Investments:    
Equity securities 1 1
Investments: | Level I    
Investments:    
Total investments 53 55
Investments: | Level I | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 53 55
Investments: | Level I | Corporate securities    
Investments:    
Fair value 0 0
Investments: | Level I | Municipal securities    
Investments:    
Fair value 0 0
Investments: | Level I | Short-term time deposits    
Investments:    
Fair value 0 0
Investments: | Level I | Asset-backed securities    
Investments:    
Fair value 0 0
Investments: | Level I | Residential mortgage-backed securities    
Investments:    
Fair value 0 0
Investments: | Level I | Commercial mortgage-backed securities    
Investments:    
Fair value 0 0
Investments: | Level I | Equity securities    
Investments:    
Equity securities 0 0
Investments: | Level II    
Investments:    
Total investments 17,917 18,280
Investments: | Level II | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 0 0
Investments: | Level II | Corporate securities    
Investments:    
Fair value 10,558 10,652
Investments: | Level II | Municipal securities    
Investments:    
Fair value 2,860 2,906
Investments: | Level II | Short-term time deposits    
Investments:    
Fair value 68 205
Investments: | Level II | Asset-backed securities    
Investments:    
Fair value 1,612 1,666
Investments: | Level II | Residential mortgage-backed securities    
Investments:    
Fair value 1,703 1,714
Investments: | Level II | Commercial mortgage-backed securities    
Investments:    
Fair value 1,115 1,136
Investments: | Level II | Equity securities    
Investments:    
Equity securities 1 1
Investments: | Level III    
Investments:    
Total investments 0 0
Investments: | Level III | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 0 0
Investments: | Level III | Corporate securities    
Investments:    
Fair value 0 0
Investments: | Level III | Municipal securities    
Investments:    
Fair value 0 0
Investments: | Level III | Short-term time deposits    
Investments:    
Fair value 0 0
Investments: | Level III | Asset-backed securities    
Investments:    
Fair value 0 0
Investments: | Level III | Residential mortgage-backed securities    
Investments:    
Fair value 0 0
Investments: | Level III | Commercial mortgage-backed securities    
Investments:    
Fair value 0 0
Investments: | Level III | Equity securities    
Investments:    
Equity securities 0 0
Restricted deposits:    
Investments:    
Fair value 1,432 1,412
Restricted deposits: | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 479 480
Restricted deposits: | Corporate securities    
Investments:    
Fair value 34 10
Restricted deposits: | Municipal securities    
Investments:    
Fair value 829 852
Restricted deposits: | Cash and cash equivalents    
Investments:    
Fair value 89 69
Restricted deposits: | Certificates of deposit    
Investments:    
Fair value 1 1
Restricted deposits: | Level I    
Investments:    
Fair value 568 549
Restricted deposits: | Level I | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 479 480
Restricted deposits: | Level I | Corporate securities    
Investments:    
Fair value 0 0
Restricted deposits: | Level I | Municipal securities    
Investments:    
Fair value 0 0
Restricted deposits: | Level I | Cash and cash equivalents    
Investments:    
Fair value 89 69
Restricted deposits: | Level I | Certificates of deposit    
Investments:    
Fair value 0 0
Restricted deposits: | Level II    
Investments:    
Fair value 864 863
Restricted deposits: | Level II | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 0 0
Restricted deposits: | Level II | Corporate securities    
Investments:    
Fair value 34 10
Restricted deposits: | Level II | Municipal securities    
Investments:    
Fair value 829 852
Restricted deposits: | Level II | Cash and cash equivalents    
Investments:    
Fair value 0 0
Restricted deposits: | Level II | Certificates of deposit    
Investments:    
Fair value 1 1
Restricted deposits: | Level III    
Investments:    
Fair value 0 0
Restricted deposits: | Level III | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Fair value 0 0
Restricted deposits: | Level III | Corporate securities    
Investments:    
Fair value 0 0
Restricted deposits: | Level III | Municipal securities    
Investments:    
Fair value 0 0
Restricted deposits: | Level III | Cash and cash equivalents    
Investments:    
Fair value 0 0
Restricted deposits: | Level III | Certificates of deposit    
Investments:    
Fair value $ 0 $ 0