Workers' Compensation Loss Reserves - Summary of Activities in Liability for Unpaid Claims and Claims Adjustment Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense
Liability for unpaid claims and claims adjustment at beginning of period	$ 190,102	$ 148,034	$ 120,739
Incurred
Current year	112,967	89,137	63,377
Prior years	28,243	26,391	15,401
Total incurred	141,210	115,528	78,778
Paid
Current year	(14,411)	(16,376)	(13,086)
Prior years	(62,287)	(57,084)	(38,397)
Total paid	(76,698)	(73,460)	(51,483)
Liability for unpaid claims and claims adjustment at end of period	254,614	190,102	148,034
Collateral paid to carriers and offset against loss reserves	(31,611)	(26,890)	(55,492)
Payable in more than 1 year (net of collateral paid to carriers of $22,377, $15,129 and $45,217 as of December 31, 2016, 2015 and 2014, respectively)	159,301	105,481
Workers' Compensation Loss Reserves	223,003	163,212	92,542
Collateral paid to insurance carriers, net of workers' compensation loss reserves, noncurrent	22,377	15,129
WSE
Paid
Payable in less than 1 year (net of collateral paid to carriers of $9,234, $11,761 and $10,275 as of December 31, 2016, 2015 and 2014, respectively)	63,702	57,731	17,094
Payable in more than 1 year (net of collateral paid to carriers of $22,377, $15,129 and $45,217 as of December 31, 2016, 2015 and 2014, respectively)	159,301	105,481	75,448
Workers' Compensation Loss Reserves	223,003	163,212	92,542
Collateral paid to insurance carriers, net of workers' compensation loss reserves, current	9,234	11,761	10,275
Collateral paid to insurance carriers, net of workers' compensation loss reserves, noncurrent	$ 22,377	$ 15,129	$ 45,217