Cash, Cash Equivalents and Investments - Summary of Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 330	$ 336
Restricted cash, cash equivalents and investments	699	1,280
Restricted cash, cash equivalents and investments, noncurrent	182	162
Total restricted cash equivalents and investments	1,211	1,778
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	330	336
Restricted cash, cash equivalents and investments	696	1,277
Total restricted cash equivalents and investments	1,185	1,738
Available for sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Restricted cash, cash equivalents and investments	1	1
Total restricted cash equivalents and investments	24	38
Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Restricted cash, cash equivalents and investments	2	2
Total 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	523	1,095
Payroll funds collected | Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	523	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	71	69
Collateral for health benefits claims | Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	71	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	182	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	159	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	23	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