Cash, Cash Equivalents and Investments - Unrestricted and Restricted - Additional Information (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 500	$ 301
Investments	71	57
Total restricted cash, cash equivalents and investments	1,081	1,388
Investments, noncurrent	130	138
Collateral for workers' compensation claims	184	210
Total cash equivalents and investments and restricted cash equivalents and investments	1,966	2,094
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	500	301
Investments	0	0
Total restricted cash, cash equivalents and investments	991	1,306
Investments, noncurrent	0	0
Total cash equivalents and investments and restricted cash equivalents and investments	1,521	1,643
Available-for-sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	71	57
Total restricted cash, cash equivalents and investments	90	82
Investments, noncurrent	130	138
Total cash equivalents and investments and restricted cash equivalents and investments	445	451
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	906	1,228
Payroll funds collected | Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Total restricted cash, cash equivalents and investments	906	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	22	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	90	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	184	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	30	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	154	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