Cash, Cash Equivalents and Investments - Unrestricted and Restricted - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 525	$ 301
Investments	155	57
Total restricted cash, cash equivalents and investments	1,040	1,388
Investments, noncurrent	167	138
Collateral for workers' compensation claims	176	210
Total cash equivalents and investments and restricted cash equivalents and investments	2,063	2,094
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	525	301
Investments	0	0
Total restricted cash, cash equivalents and investments	946	1,306
Investments, noncurrent	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	1,502	1,643
Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	155	57
Total restricted cash, cash equivalents and investments	94	82
Investments, noncurrent	167	138
Total cash equivalents and investments and restricted cash equivalents and investments	561	451
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	863	1,228
Payroll funds collected | Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	863	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	118	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	24	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	94	82
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	57	60
Collateral for workers' compensation claims	176	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	57	60
Collateral for workers' compensation claims	31	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	145	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