Other Accrued Expenses Other Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Other Accrued Expenses [Abstract]
Accrued Employee Benefits, Current	$ 14,401	$ 22,282
Asset Retirement Obligation, Current	(13,128)	(9,775)
Other Accrued Liabilities	5,922	6,568
Other Accrued Liabilities, Noncurrent	$ 33,451	$ 38,625