Description of Business and Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policy [Line Items]
Number of reportable segments as result of acquisitions | segment	1
Gross payroll and payroll tax payments made	$ 34,300,000,000	$ 30,600,000,000	$ 25,600,000,000
Reimbursement per claim occurrence (up to)	$ 1,000,000
Plan years to be settled (in years)	10 years
Prepaid insurance premiums	$ 8,600,000	6,800,000
Internally developed software costs	21,300,000	11,200,000	6,300,000
Impairment loss	0	0	0
Impairment of intangible assets (excluding goodwill)	0	0	0
Advertising costs	6,400,000	8,200,000	7,300,000
Bad debt expense, net of recoveries	1,300,000	2,000,000	1,400,000
Debt issuance costs	4,018,000	5,781,000
Non-cancelable operating lease commitments	64,644,000
Net cash provided by operating activities	144,532,000	130,599,000	151,899,000
Net cash used in financing activities	(99,371,000)	(60,752,000)	$ (66,372,000)
Restricted cash, cash equivalents and payroll funds collected	$ 1,000,000,000	$ 1,000,000,000
Customer Concentration Risk | Accounts Receivable
Summary Of Significant Accounting Policy [Line Items]
Concentration risk, percentage		12.00%
Minimum
Summary Of Significant Accounting Policy [Line Items]
Useful life of finite-lived intangible assets	2 years
Maximum
Summary Of Significant Accounting Policy [Line Items]
Useful life of finite-lived intangible assets	10 years
Maximum | Geographic Concentration Risk | Sales revenue, net
Summary Of Significant Accounting Policy [Line Items]
Concentration risk, percentage	1.00%
Software And Office Equipment | Minimum
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life	3 years
Software And Office Equipment | Maximum
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life	5 years
Furniture, fixtures, and equipment | Minimum
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life	5 years
Furniture, fixtures, and equipment | Maximum
Summary Of Significant Accounting Policy [Line Items]
Estimated useful life	7 years
Software Development | Minimum
Summary Of Significant Accounting Policy [Line Items]
Useful life of finite-lived intangible assets	3 years
Software Development | Maximum
Summary Of Significant Accounting Policy [Line Items]
Useful life of finite-lived intangible assets	5 years
Accounting Standards Update 2015-03 | Notes and Capital Leases Payable, Current
Summary Of Significant Accounting Policy [Line Items]
Debt issuance costs		$ 2,400,000
Accounting Standards Update 2015-03 | Notes and Capital Leases Payable, Noncurrent
Summary Of Significant Accounting Policy [Line Items]
Debt issuance costs		3,400,000
Accounting Standards Update 2015-03 | Other Current Assets
Summary Of Significant Accounting Policy [Line Items]
Debt issuance costs		(2,400,000)
Accounting Standards Update 2015-03 | Other Assets
Summary Of Significant Accounting Policy [Line Items]
Debt issuance costs		(3,400,000)
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2016-09
Summary Of Significant Accounting Policy [Line Items]
Net cash provided by operating activities	$ 4,600,000	20,700,000
Net cash used in financing activities	$ (4,600,000)	$ (20,700,000)
Guaranteed Cost | Product Concentration Risk | Sales Revenue, Services, Net
Summary Of Significant Accounting Policy [Line Items]
Concentration risk, percentage	38.00%
Risk-Based | Product Concentration Risk
Summary Of Significant Accounting Policy [Line Items]
Concentration risk, percentage	62.00%