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	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Financial Instruments [Line Items]
Total investments	$ 445	$ 451
Total cash equivalents and investments and restricted cash equivalents and investments	1,966	2,094
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	127	13
Total investments	202	195
Total restricted cash equivalents	96	99
Total restricted investments	243	256
Total cash equivalents and investments and restricted cash equivalents and investments	668	563
Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	127	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	222	101
Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	0	11
Total investments	202	195
Total restricted cash equivalents	1	0
Total restricted investments	243	256
Total cash equivalents and investments and restricted cash equivalents and investments	446	462
Money market mutual funds | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total cash equivalents	127	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	127	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		11
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total cash equivalents		0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total cash equivalents		11
Certificate of deposits | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	1
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	1
Commercial paper | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 1
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 2
Fair Value, Financial Instruments [Line Items]
Total restricted cash equivalents	0
Asset-backed securities
Fair Value, Financial Instruments [Line Items]
Total investments	25	24
Asset-backed securities | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	25	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	25	24
Corporate bonds
Fair Value, Financial Instruments [Line Items]
Total investments	123	128
Corporate bonds | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	89	93
Total restricted investments	34	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	89	93
Total restricted investments	34	35
U.S. government agencies and government- sponsored agencies
Fair Value, Financial Instruments [Line Items]
Total investments	23	28
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	5	5
Total restricted investments	18	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	5	5
Total restricted investments	18	23
U.S. treasuries
Fair Value, Financial Instruments [Line Items]
Total investments	259	265
U.S. treasuries | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	77	67
Total restricted investments	182	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	77	67
Total restricted investments	182	198
Certificate of deposits
Fair Value, Financial Instruments [Line Items]
Total investments	9	0
Certificate of deposits | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total restricted investments	9
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	9
Other debt securities
Fair Value, Financial Instruments [Line Items]
Total investments	6	6
Other debt securities | Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Total investments	6	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	$ 6	$ 6