Property, Plant and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Amortization expense on assets recorded under capital leases	$ 1,500	$ 1,700	$ 1,700
Accumulated depreciation and amortization, capital lease assets	242,623	231,062
Related Party
Property, Plant and Equipment [Line Items]
Accumulated depreciation and amortization, capital lease assets	3,600	9,000
Non-Related Party
Property, Plant and Equipment [Line Items]
Accumulated depreciation and amortization, capital lease assets	$ 4,700	$ 6,900