Workers Compensation Assets and Liabilities - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Liability for Unpaid Claims and Claims Adjustment Expense
Total loss reserves, beginning of period	$ 260,308	$ 200,152	$ 254,614	$ 190,102
Incurred
Current year	20,191	28,572	47,127	51,346
Prior years	896	2,741	1,497	8,004
Total incurred	21,087	31,313	48,624	59,350
Paid
Current year	(5,377)	(1,567)	(5,637)	(2,278)
Prior years	(21,334)	(15,187)	(42,917)	(32,463)
Total paid	(26,711)	(16,754)	(48,554)	(34,741)
Total loss reserves, end of period	$ 254,684	$ 214,711	$ 254,684	$ 214,711