Accrued Liabilities Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities [Abstract]
Accrued insurance costs	$ 124,760	$ 125,172
Deferred revenue	51,742	58,255
Accrued compensation	37,783	40,551
Taxes other than income	27,415	22,556
Deferred compensation	11,323	11,394
Accrued interest payable	5,707	5,808
Deferred rent	3,937	5,199
Accrued dividends	0	4,262
Accrued other	35,351	58,999
Accrued liabilities	$ 298,018	$ 332,196