Financial Instruments and Fair Value Measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total	$ 1,798	$ 1,546
Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Total	1	2
Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	92	5
Investments	194	189
Restricted cash equivalents	70	68
Restricted investments	148	8
Total	504	270
Fair Value Measurements on a Recurring Basis \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Investments	30	33
Fair Value Measurements on a Recurring Basis \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Investments	96	99
Restricted investments	28
Fair Value Measurements on a Recurring Basis \| U.S. government agencies and government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Investments	5	7
Restricted investments	9
Fair Value Measurements on a Recurring Basis \| U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Investments	53	41
Restricted investments	110	5
Fair Value Measurements on a Recurring Basis \| Other debt securities
Debt Securities, Available-for-sale [Line Items]
Investments	10	9
Fair Value Measurements on a Recurring Basis \| Exchange traded fund
Debt Securities, Available-for-sale [Line Items]
Restricted investments		1
Fair Value Measurements on a Recurring Basis \| Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	2
Restricted investments	1	2
Fair Value Measurements on a Recurring Basis \| Level 1
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	89	4
Investments	0	0
Restricted cash equivalents	56	68
Restricted investments	0	1
Total	145	73
Fair Value Measurements on a Recurring Basis \| Level 1 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Investments	0	0
Fair Value Measurements on a Recurring Basis \| Level 1 \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Investments	0	0
Restricted investments	0
Fair Value Measurements on a Recurring Basis \| Level 1 \| U.S. government agencies and government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Investments	0	0
Restricted investments	0
Fair Value Measurements on a Recurring Basis \| Level 1 \| U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Investments	0	0
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis \| Level 1 \| Other debt securities
Debt Securities, Available-for-sale [Line Items]
Investments	0	0
Fair Value Measurements on a Recurring Basis \| Level 1 \| Exchange traded fund
Debt Securities, Available-for-sale [Line Items]
Restricted investments		1
Fair Value Measurements on a Recurring Basis \| Level 1 \| Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	0
Restricted investments	0	0
Fair Value Measurements on a Recurring Basis \| Level 2
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	3	1
Investments	194	189
Restricted cash equivalents	14	0
Restricted investments	148	7
Total	359	197
Fair Value Measurements on a Recurring Basis \| Level 2 \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Investments	30	33
Fair Value Measurements on a Recurring Basis \| Level 2 \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Investments	96	99
Restricted investments	28
Fair Value Measurements on a Recurring Basis \| Level 2 \| U.S. government agencies and government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Investments	5	7
Restricted investments	9
Fair Value Measurements on a Recurring Basis \| Level 2 \| U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Investments	53	41
Restricted investments	110	5
Fair Value Measurements on a Recurring Basis \| Level 2 \| Other debt securities
Debt Securities, Available-for-sale [Line Items]
Investments	10	9
Fair Value Measurements on a Recurring Basis \| Level 2 \| Exchange traded fund
Debt Securities, Available-for-sale [Line Items]
Restricted investments		0
Fair Value Measurements on a Recurring Basis \| Level 2 \| Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	2
Restricted investments	1	2
Money market mutual funds \| Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	89	4
Restricted cash equivalents	42	48
Money market mutual funds \| Fair Value Measurements on a Recurring Basis \| Level 1
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	89	4
Restricted cash equivalents	42	48
Money market mutual funds \| Fair Value Measurements on a Recurring Basis \| Level 2
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	0	0
Restricted cash equivalents	0	0
U.S. treasuries \| Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	3	1
Restricted cash equivalents	12
U.S. treasuries \| Fair Value Measurements on a Recurring Basis \| Level 1
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	0	0
Restricted cash equivalents	0
U.S. treasuries \| Fair Value Measurements on a Recurring Basis \| Level 2
Debt Securities, Available-for-sale [Line Items]
Cash equivalents	3	1
Restricted cash equivalents	12
Commercial Paper \| Fair Value Measurements on a Recurring Basis
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	14	20
Commercial Paper \| Fair Value Measurements on a Recurring Basis \| Level 1
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	14	20
Commercial Paper \| Fair Value Measurements on a Recurring Basis \| Level 2
Debt Securities, Available-for-sale [Line Items]
Restricted cash equivalents	$ 0	$ 0

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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