Cash, Cash Equivalents and Investments - Additional Information (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 219	$ 228
Investments	76	54
Restricted cash, cash equivalents and investments	672	942
Investments, noncurrent	117	135
Restricted cash, cash equivalents and investments, noncurrent	200	187
Total unrestricted and restricted cash equivalents and investments	1,284	1,546
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	219	228
Investments	0	0
Restricted cash, cash equivalents and investments	671	939
Investments, noncurrent	0	0
Total unrestricted and restricted cash equivalents and investments	1,085	1,349
Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	76	54
Restricted cash, cash equivalents and investments	1	1
Investments, noncurrent	117	135
Total unrestricted and restricted cash equivalents and investments	199	195
Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		0
Investments		0
Restricted cash, cash equivalents and investments		2
Investments, noncurrent		0
Total unrestricted and restricted cash equivalents and investments		2
Insurance carriers' security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	14	15
Insurance carriers' security deposits \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	14	15
Insurance carriers' security deposits \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Insurance carriers' security deposits \| Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	519	783
Payroll funds collected \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	519	783
Payroll funds collected \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Payroll funds collected \| Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	75	75
Collateral for health benefits claims \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	75	75
Collateral for health benefits claims \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Collateral for health benefits claims \| Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	64	67
Restricted cash, cash equivalents and investments, noncurrent	200	187
Collateral for workers' compensation claims \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	63	66
Restricted cash, cash equivalents and investments, noncurrent	195	182
Collateral for workers' compensation claims \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	1	1
Restricted cash, cash equivalents and investments, noncurrent	5	5
Collateral for workers' compensation claims \| Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		0
Restricted cash, cash equivalents and investments, noncurrent		0
Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	2
Collateral to secure standby letter of credit \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Collateral to secure standby letter of credit \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	$ 0	0
Collateral to secure standby letter of credit \| Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments		$ 2

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

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