Financial Instruments and Fair Value Measurements - Summary of Financial Instruments by Significant Categories and Fair Value Measurement on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Financial Instruments [Line Items]
Total investments	$ 561	$ 451
Total cash equivalents and investments and restricted cash equivalents and investments	2,063	2,094
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	5	13
Total investments	322	195
Total restricted cash equivalents	96	99
Total restricted investments	239	256
Total cash equivalents and investments and restricted cash equivalents and investments	662	563
Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	2	2
Total investments	0	0
Total restricted cash equivalents	95	99
Total restricted investments	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	97	101
Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	11
Total investments	322	195
Total restricted cash equivalents	1	0
Total restricted investments	239	256
Total cash equivalents and investments and restricted cash equivalents and investments	565	462
Money market mutual funds | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	2	2
Total restricted cash equivalents	95	99
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	2	2
Total restricted cash equivalents	95	99
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	0	0
Total restricted cash equivalents	0	0
U.S. treasuries | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	11
Total restricted cash equivalents	1
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	0	0
Total restricted cash equivalents	0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	3	11
Total restricted cash equivalents	1
Asset-backed securities
Fair Value, Financial Instruments [Line Items]
Total investments	48	24
Asset-backed securities | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	48	24
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	48	24
Corporate bonds
Fair Value, Financial Instruments [Line Items]
Total investments	172	128
Corporate bonds | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	140	93
Total restricted investments	32	35
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Total restricted investments	0	0
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	140	93
Total restricted investments	32	35
U.S. government agencies and government- sponsored agencies
Fair Value, Financial Instruments [Line Items]
Total investments	18	28
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	2	5
Total restricted investments	16	23
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Total restricted investments	0	0
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	2	5
Total restricted investments	16	23
U.S. treasuries
Fair Value, Financial Instruments [Line Items]
Total investments	301	265
U.S. treasuries | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	122	67
Total restricted investments	179	198
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Total restricted investments	0	0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	122	67
Total restricted investments	179	198
Other debt securities
Fair Value, Financial Instruments [Line Items]
Total investments	10	6
Other debt securities | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	10	6
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total investments	0	0
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total investments	10	6
Certificate of deposits
Fair Value, Financial Instruments [Line Items]
Total investments	12	$ 0
Certificate of deposits | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted investments	12
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total restricted investments	0
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total restricted investments	$ 12