Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Property, plant and equipment
Land		$ 9,127	$ 9,408
Building improvements		559,454	554,160
Capitalized leases for machinery, equipment and buildings		99,228	82,753
Property, plant and equipment, gross		667,809	646,321
Accumulated depreciation and amortization	[1]	(464,812)	(448,001)
Property Plant and Equipment Net before Construction in Progress		202,997	198,320
Construction in Progress		70,340	56,417
Property, plant and equipment, net		$ 273,337	$ 254,737

[1] Includes $39.7 million and $28.2 million of accumulated amortization related to assets under capital leases at June 30, 2018 and December 31, 2017, respectively.