Property, Plant and Equipment (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment
Land		$ 9,408	$ 7,869
Building improvements		554,160	532,122
Capitalized leases for machinery, equipment and buildings		82,753	54,297
Property, plant and equipment, gross		646,321	594,288
Accumulated depreciation and amortization	[1]	(448,001)	(423,361)
Property Plant and Equipment Net before Construction in Progress		198,320	170,927
Construction in Progress		56,417	22,496
Property, plant and equipment, net		$ 254,737	$ 193,423

[1] Includes $28.2 million and $14.8 million of accumulated amortization related to assets under capital leases at December 31, 2017 and 2016, respectively.