Description of Business and Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policy [Line Items]
Number of reportable segments | segment	1
Maximum reimbursement per claim occurrence	$ 1,000,000
Plan years to be settled	10 years
Prepaid insurance premiums	$ 11,000,000	$ 9,000,000
Internally developed software costs	29,000,000	21,000,000	$ 11,000,000
Impairment of goodwill	0	0	0
Impairment of intangible assets	0	0	0
Advertising costs	8,000,000	6,000,000	8,000,000
Bad debt expense, net of recoveries	1,000,000	1,000,000	2,000,000
Reclassification of tax deficiencies from financing activities	77,000,000	104,000,000	81,000,000
Reclassification of tax deficiencies to operating activities	253,000,000	149,000,000	151,000,000
Restricted cash, cash equivalents and payroll funds collected	$ 1,400,000,000	1,000,000,000
ASU 2016-09
Summary Of Significant Accounting Policy [Line Items]
Reclassification of tax deficiencies from financing activities		5,000,000	21,000,000
Reclassification of tax deficiencies to operating activities		$ 5,000,000	$ 21,000,000
Minimum
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of intangible assets	2 years
Minimum | Software Development
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of intangible assets	3 years
Minimum | Software And Office Equipment
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of property and equipment	3 years
Minimum | Furniture, fixtures, and equipment
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of property and equipment	5 years
Maximum
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of intangible assets	10 years
Maximum | Software Development
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of intangible assets	5 years
Maximum | Software And Office Equipment
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of property and equipment	5 years
Maximum | Furniture, fixtures, and equipment
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life of property and equipment	7 years
Geographic Concentration Risk | Revenue | Maximum
Summary Of Significant Accounting Policy [Line Items]
Concentration risk percentage	1.00%
Customer Concentration Risk | Accounts Receivable | Largest Customer
Summary Of Significant Accounting Policy [Line Items]
Concentration risk percentage	47.00%