Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Insurance and claims accruals	$ 41,986	$ 74,015
Compensation - related accruals	6,797	10,056
Allowance for uncollectible accounts	3,599	6,135
Other	1,979	4,168
Gross deferred income tax assets	54,361	94,374
Property and equipment	243,482	377,093
Prepaid expenses	4,699	7,737
Other	1,367	2,313
Gross deferred income tax liabilities	249,548	387,143
Net deferred income tax liability	$ 195,187	$ 292,769