Income Taxes - Components of Deferred Income Tax Amounts (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Income Tax Assets
Provision for bad debts	$ 6,794	$ 10,510
Deferred compensation and other benefit obligations	68,101	112,811
Workers’ compensation	3,127	5,634
Stock-based compensation	8,614	16,772
Credits and net operating loss carryforwards	30,087	30,534
Other	13,343	18,116
Total deferred income tax assets	130,066	194,377
Deferred Income Tax Liabilities
Amortization of intangible assets	(20,220)	(28,681)
Property and equipment basis differences	(4,421)	(16,640)
Other	(10,847)	(11,658)
Total deferred income tax liabilities	(35,488)	(56,979)
Valuation allowance	(20,178)	(18,907)
Total deferred income tax assets, net	$ 74,400	$ 118,491