Fair Value Measurements - Fair Value Measurements by Level for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Assets
Cash and cash equivalents	$ 17,170	$ 12,074
Investments:
Equity securities	2	5
Total assets at fair value	35,857	29,598
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	450	679
Corporate securities
Investments:
Fair Value	9,867	9,361
Municipal securities
Investments:
Fair Value	3,931	3,781
Short-term time deposits
Investments:
Fair Value	419	204
Asset-backed securities
Investments:
Fair Value	1,448	1,326
Residential mortgage-backed securities
Investments:
Fair Value	1,227	1,046
Commercial mortgage-backed securities
Investments:
Fair Value	981	862
Investments
Investments:
Fair Value	17,330	16,302
Total investments	17,332	16,307
Investments | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	112	371
Investments | Corporate securities
Investments:
Fair Value	9,833	9,328
Investments | Municipal securities
Investments:
Fair Value	3,310	3,165
Investments | Short-term time deposits
Investments:
Fair Value	419	204
Investments | Asset-backed securities
Investments:
Fair Value	1,448	1,326
Investments | Residential mortgage-backed securities
Investments:
Fair Value	1,227	1,046
Investments | Commercial mortgage-backed securities
Investments:
Fair Value	981	862
Investments | Equity securities
Investments:
Equity securities	2	5
Restricted deposits
Investments:
Fair Value	1,355	1,217
Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	338	308
Restricted deposits | Corporate securities
Investments:
Fair Value	34	33
Restricted deposits | Municipal securities
Investments:
Fair Value	621	616
Restricted deposits | Cash and cash equivalents
Investments:
Fair Value	358	256
Restricted deposits | Certificates of deposit
Investments:
Fair Value	4	4
Level I
Assets
Cash and cash equivalents	17,170	12,074
Investments:
Total assets at fair value	17,978	13,007
Level I | Investments
Investments:
Total investments	112	369
Level I | Investments | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	112	366
Level I | Investments | Corporate securities
Investments:
Fair Value	0	0
Level I | Investments | Municipal securities
Investments:
Fair Value	0	0
Level I | Investments | Short-term time deposits
Investments:
Fair Value	0	0
Level I | Investments | Asset-backed securities
Investments:
Fair Value	0	0
Level I | Investments | Residential mortgage-backed securities
Investments:
Fair Value	0	0
Level I | Investments | Commercial mortgage-backed securities
Investments:
Fair Value	0	0
Level I | Investments | Equity securities
Investments:
Equity securities	0	3
Level I | Restricted deposits
Investments:
Fair Value	696	564
Level I | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	338	308
Level I | Restricted deposits | Corporate securities
Investments:
Fair Value	0	0
Level I | Restricted deposits | Municipal securities
Investments:
Fair Value	0	0
Level I | Restricted deposits | Cash and cash equivalents
Investments:
Fair Value	358	256
Level I | Restricted deposits | Certificates of deposit
Investments:
Fair Value	0	0
Level II
Assets
Cash and cash equivalents	0	0
Investments:
Total assets at fair value	17,879	16,591
Level II | Investments
Investments:
Total investments	17,220	15,938
Level II | Investments | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	0	5
Level II | Investments | Corporate securities
Investments:
Fair Value	9,833	9,328
Level II | Investments | Municipal securities
Investments:
Fair Value	3,310	3,165
Level II | Investments | Short-term time deposits
Investments:
Fair Value	419	204
Level II | Investments | Asset-backed securities
Investments:
Fair Value	1,448	1,326
Level II | Investments | Residential mortgage-backed securities
Investments:
Fair Value	1,227	1,046
Level II | Investments | Commercial mortgage-backed securities
Investments:
Fair Value	981	862
Level II | Investments | Equity securities
Investments:
Equity securities	2	2
Level II | Restricted deposits
Investments:
Fair Value	659	653
Level II | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	0	0
Level II | Restricted deposits | Corporate securities
Investments:
Fair Value	34	33
Level II | Restricted deposits | Municipal securities
Investments:
Fair Value	621	616
Level II | Restricted deposits | Cash and cash equivalents
Investments:
Fair Value	0	0
Level II | Restricted deposits | Certificates of deposit
Investments:
Fair Value	4	4
Level III
Assets
Cash and cash equivalents	0	0
Investments:
Total assets at fair value	0	0
Level III | Investments
Investments:
Total investments	0	0
Level III | Investments | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	0	0
Level III | Investments | Corporate securities
Investments:
Fair Value	0	0
Level III | Investments | Municipal securities
Investments:
Fair Value	0	0
Level III | Investments | Short-term time deposits
Investments:
Fair Value	0	0
Level III | Investments | Asset-backed securities
Investments:
Fair Value	0	0
Level III | Investments | Residential mortgage-backed securities
Investments:
Fair Value	0	0
Level III | Investments | Commercial mortgage-backed securities
Investments:
Fair Value	0	0
Level III | Investments | Equity securities
Investments:
Equity securities	0	0
Level III | Restricted deposits
Investments:
Fair Value	0	0
Level III | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies
Investments:
Fair Value	0	0
Level III | Restricted deposits | Corporate securities
Investments:
Fair Value	0	0
Level III | Restricted deposits | Municipal securities
Investments:
Fair Value	0	0
Level III | Restricted deposits | Cash and cash equivalents
Investments:
Fair Value	0	0
Level III | Restricted deposits | Certificates of deposit
Investments:
Fair Value	$ 0	$ 0