Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 202	$ 336
Investments	29	0
Restricted cash, cash equivalents and investments	629	1,280
Investments, noncurrent	133	0
Restricted cash, cash equivalents and investments, noncurrent	179	162
Total investments and restricted cash equivalents and investments	1,172	1,778
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	202	336
Investments	0	0
Restricted cash, cash equivalents and investments	626	1,277
Investments, noncurrent	0	0
Total investments and restricted cash equivalents and investments	993	1,738
Available for sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	29	0
Restricted cash, cash equivalents and investments	1	1
Investments, noncurrent	133	0
Total investments and restricted cash equivalents and investments	177	38
Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	0	0
Restricted cash, cash equivalents and investments	2	2
Investments, noncurrent	0	0
Total investments and restricted cash equivalents and investments	2	2
Insurance carriers security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	15	15
Insurance carriers security deposits \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	15	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	0
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	449	1,095
Payroll funds collected \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	449	1,095
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	0
Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	75	69
Collateral for health benefits claims \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	75	69
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	0
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	88	99
Restricted cash, cash equivalents and investments, noncurrent	179	162
Collateral for workers' compensation claims \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	87	98
Restricted cash, cash equivalents and investments, noncurrent	165	125
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	14	37
Collateral for workers' compensation claims \| Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Restricted cash, cash equivalents and investments, noncurrent	0	0
Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	2	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	$ 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 current.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section S99
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