Business Combinations - Additional Information (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) acquisition	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Cash payments to acquire business		$ 4,750	$ 0	$ 194,998
Intangible assets acquired	$ 4,400
Goodwill acquired	$ 400
Useful life of intangible assets (in years)	5 years
Number of businesses acquired | acquisition			0
Ambrose Employer Group LLC
Business Acquisition [Line Items]
Percentage of equity acquired				100.00%
Consideration transferred				$ 195,000
Revenue from acquired entity				134,500
Net Income (loss) from acquired entity since acquisition date				$ 1,600