Description of Business and Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) customer segment	Dec. 31, 2014 USD ($) customer	Dec. 31, 2013 USD ($) customer
Summary Of Significant Accounting Policy [Line Items]
Number of reportable segments as result of acquisitions | segment	1
Gross payroll and payroll tax payments made	$ 30,600,000,000	$ 25,600,000,000	$ 17,600,000,000
Workers compensation program time until settlement (more than)	10 years
Health benefits payable	$ 113,200,000	82,100,000
Prepaid health plan expenses	$ 6,800,000	$ 4,900,000
Useful life of finite-lived intangible assets	5 years	5 years
Internally developed software costs	$ 11,200,000	$ 6,300,000	3,300,000
Impairment loss	0	0
Advertising costs	8,200,000	7,300,000	7,500,000
Customer Concentration Risk
Summary Of Significant Accounting Policy [Line Items]
Bad debt expense, net of recoveries	$ 2,000,000	$ 1,400,000	$ 600,000
Customer Concentration Risk | Accounts Receivable
Summary Of Significant Accounting Policy [Line Items]
Concentration risk, percentage	12.00%
Number of customers accounted for more than 10% | customer	1	1
Customer Concentration Risk | Service Revenues
Summary Of Significant Accounting Policy [Line Items]
Concentration risk, percentage	10.00%
Number of customers accounted for more than 10% | customer	0	0	0
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	6 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