Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Property, plant and equipment
Land		$ 9,107	$ 9,408
Building improvements		559,276	554,160
Capitalized leases for machinery, equipment and buildings		88,720	82,753
Property, plant and equipment, gross		657,103	646,321
Accumulated depreciation and amortization	[1]	(456,374)	(448,001)
Property Plant and Equipment Net before Construction in Progress		200,729	198,320
Construction in Progress		65,428	56,417
Property, plant and equipment, net		$ 266,157	$ 254,737

[1] Includes $29.0 million and $28.2 million of accumulated amortization related to assets under capital leases at March 31, 2018 and December 31, 2017, respectively.