Cash, Cash Equivalents and Investments - Unrestricted and Restricted - Additional Information (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 464	$ 301
Investments	131	57
Total restricted cash, cash equivalents and investments	958	1,388
Investments, noncurrent	194	138
Collateral for workers' compensation claims	179	210
Total cash equivalents and investments and restricted cash equivalents and investments	1,926	2,094
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	464	301
Investments	0	0
Total restricted cash, cash equivalents and investments	869	1,306
Investments, noncurrent	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	1,361	1,643
Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	131	57
Total restricted cash, cash equivalents and investments	89	82
Investments, noncurrent	194	138
Total cash equivalents and investments and restricted cash equivalents and investments	565	451
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	783	1,228
Payroll funds collected \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	783	1,228
Payroll funds collected \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	0	0
Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	112	98
Collateral for health benefits claims \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	23	16
Collateral for health benefits claims \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	89	82
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	61	60
Collateral for workers' compensation claims	179	210
Collateral for workers' compensation claims \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	61	60
Collateral for workers' compensation claims	28	36
Collateral for workers' compensation claims \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	0	0
Collateral for workers' compensation claims	151	174
Other security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	2	2
Other security deposits \| Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	2	2
Other security deposits \| Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Total 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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The noncurrent cash, cash equivalents and investments that is restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits classified as long-term; that is not expected to be released from such existing restrictions within one year of the balance sheet date or operating cycle, whichever is longer. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes noncurrent cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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