Property, Plant and Equipment - Narratives (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Impaired tangible assets	$ 0	$ 14,956,000	$ 0
Depreciation	$ 12,849,000	$ 13,917,000	$ 14,429,000