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, 2020	Dec. 31, 2019
Fair Value, Financial Instruments [Line Items]
Investments	$ 460	$ 342
Total cash equivalents and investments and restricted cash equivalents and investments	1,802	1,798
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7	92
Investments	202	194
Restricted cash equivalents	97	70
Restricted investments	258	148
Total cash equivalents and investments and restricted cash equivalents and investments	564	504
Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7	89
Investments	0	0
Restricted cash equivalents	97	56
Restricted investments	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	104	145
Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0	3
Investments	202	194
Restricted cash equivalents	0	14
Restricted investments	258	148
Total cash equivalents and investments and restricted cash equivalents and investments	460	359
Money market mutual funds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7	89
Restricted cash equivalents	84	42
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	7	89
Restricted cash equivalents	84	42
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0	0
Restricted cash equivalents	0	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents		3
Restricted cash equivalents		12
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents		0
Restricted cash equivalents		0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents		3
Restricted cash equivalents		12
Certificate of deposit \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents		2
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents		0
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents		2
Commercial paper \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	13	14
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	13	14
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	0	0
Asset-backed securities
Fair Value, Financial Instruments [Line Items]
Investments	22	30
Asset-backed securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	22	30
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Asset-backed securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	22	30
Corporate bonds
Fair Value, Financial Instruments [Line Items]
Investments	121	124
Corporate bonds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	90	96
Restricted investments	31	28
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Restricted investments	0	0
Corporate bonds \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	90	96
Restricted investments	31	28
U.S. government agencies and government- sponsored agencies
Fair Value, Financial Instruments [Line Items]
Investments	20	14
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	6	5
Restricted investments	14	9
U.S. government agencies and government- sponsored agencies \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
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]
Investments	6	5
Restricted investments	14	9
U.S. treasuries
Fair Value, Financial Instruments [Line Items]
Investments	290	163
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	77	53
Restricted investments	213	110
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Restricted investments	0	0
U.S. treasuries \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	77	53
Restricted investments	213	110
Other debt securities
Fair Value, Financial Instruments [Line Items]
Investments	7	10
Other debt securities \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	7	10
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Investments	0	0
Other debt securities \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Investments	7	10
Certificate of deposit
Fair Value, Financial Instruments [Line Items]
Investments	$ 0	1
Certificate of deposit \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Restricted investments		1
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted investments		0
Certificate of deposit \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted investments		$ 1

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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