Income Taxes - Components of Deferred Income Tax Amounts (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Income Tax Assets
Deferred compensation and other benefit obligations	$ 87,513	$ 68,101
Credits and net operating loss carryforwards	31,169	30,087
Stock-based compensation	9,535	8,614
Provision for bad debts	7,891	6,794
Workers’ compensation	3,580	3,127
Other	14,959	13,343
Total deferred income tax assets	154,647	130,066
Deferred Income Tax Liabilities
Amortization of intangible assets	(21,210)	(20,220)
Property and equipment basis differences	(9,761)	(4,421)
Other	(10,319)	(10,847)
Total deferred income tax liabilities	(41,290)	(35,488)
Valuation allowance	(23,072)	(20,178)
Total deferred income tax assets, net	$ 90,285	$ 74,400