Income Taxes Deferred tax assets and liabilties (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Abstract]
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts	$ 515	$ 559
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	6,550	9,275
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost	156	254
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Self Insurance	18,225	29,190
Deferred Tax Assets, Operating Loss Carryforwards	237	451
Deferred Tax Assets, Tax Deferred Expense	1,278	3,334
Deferred Tax Assets, Other	0	2
Deferred Tax Assets, Gross	26,961	43,065
Deferred Tax Assets, Valuation Allowance	0	0
Deferred Tax Assets, Net	26,961	43,065
Deferred Tax Liabilities, Property, Plant and Equipment	(81,322)	(122,367)
Deferred Tax Liabilities, Goodwill and Intangible Assets	(7,984)	(9,857)
Deferred Tax Liabilities, Prepaid Expenses	1,406	1,713
Deferred Income Tax Liabilities	(90,712)	(133,937)
Deferred Tax Liabilities, Net	$ (63,751)	$ (90,872)