Long-Lived Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property, plant and equipment
Property, plant and equipment cost	$ 1,151,184		$ 1,131,271
Less accumulated depreciation and amortization	(544,097)		(522,973)
Total property, plant and equipment, net	607,087		$ 608,298
Depreciation expense	$ 23,000	$ 20,800