Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 213	$ 228
Short-term Investments	68	54
Restricted cash, cash equivalents and investments	1,180	942
Investments, noncurrent	125	135
Collateral for workers' compensation claims	212	187
Total	1,798	1,546
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	1,018	783
Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	98	75
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	62	67
Collateral for workers' compensation claims	212	187
Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	2
Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	2	15
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	213	228
Short-term Investments	0	0
Restricted cash, cash equivalents and investments	1,180	939
Investments, noncurrent	0	0
Total	1,456	1,349
Cash and cash equivalents \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	1,018	783
Cash and cash equivalents \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	98	75
Cash and cash equivalents \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	62	66
Collateral for workers' compensation claims	63	182
Cash and cash equivalents \| Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Cash and cash equivalents \| Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	2	15
Available for sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Short-term Investments	68	54
Restricted cash, cash equivalents and investments	0	1
Investments, noncurrent	125	135
Total	341	195
Available for sale marketable securities \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	1
Collateral for workers' compensation claims	148	5
Available for sale marketable securities \| Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Short-term Investments	0	0
Restricted cash, cash equivalents and investments	0	2
Investments, noncurrent	0	0
Total	1	2
Certificate of deposits \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Collateral for workers' compensation claims	1	0
Certificate of deposits \| Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	2
Certificate of deposits \| Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	$ 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), classified as noncurrent.

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