Workers Compensation - Summary of Activities in Liability for Unpaid Claims and Claims Adjustment Expenses (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Insurance [Abstract]
Liability for unpaid claims and claims adjustment at beginning of period	$ 58,610	$ 53,900
Plans acquired through business combinations		481
Incurred related to:
Current year	61,669	26,401
Prior years	(4,725)	(3,319)
Total incurred	56,944	23,082
Paid related to:
Current year	(13,525)	(8,055)
Prior years	(9,623)	(10,798)
Total paid	(23,148)	(18,853)
Liability for unpaid claims and claims adjustment at end of period	92,406	58,610
Other premiums and collateral liabilities	19,820	10,152
Total workers compensation liabilities at end of period	112,226	68,762
Current portion included in worksite employee-related liability	36,778	23,453
Workers compensation liabilities	$ 75,448	$ 45,309