Property, Plant and Equipment - Depreciation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Depreciation Expense [Abstract]
Depreciation	$ 1,244	$ 1,457	$ 2,873	$ 2,884