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	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Financial Instruments [Line Items]
Investments	$ 453	$ 342
Total cash equivalents and investments and restricted cash equivalents and investments	1,780	1,798
Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	286	92
Investments	204	194
Restricted cash equivalents	105	70
Restricted investments	249	148
Total cash equivalents and investments and restricted cash equivalents and investments	844	504
Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	286	89
Investments	0	0
Restricted cash equivalents	105	56
Restricted investments	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	391	145
Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Cash equivalents	0	3
Investments	204	194
Restricted cash equivalents	0	14
Restricted investments	249	148
Total cash equivalents and investments and restricted cash equivalents and investments	453	359
Money market mutual funds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Cash equivalents	286	89
Restricted cash equivalents	91	42
Money market mutual funds \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Cash equivalents	286	89
Restricted cash equivalents	91	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	14	14
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 1
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	14	14
Commercial paper \| Fair Value Measurement on a Recurring Basis \| Level 2
Fair Value, Financial Instruments [Line Items]
Restricted cash equivalents	0	0
U.S. treasuries
Fair Value, Financial Instruments [Line Items]
Investments	285	163
U.S. treasuries \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	86	53
Restricted investments	199	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	86	53
Restricted investments	199	110
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	114	124
Corporate bonds \| Fair Value Measurement on a Recurring Basis
Fair Value, Financial Instruments [Line Items]
Investments	83	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	83	96
Restricted investments	31	28
U.S. government agencies and government- sponsored agencies
Fair Value, Financial Instruments [Line Items]
Investments	25	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	19	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	19	9
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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