Assets Manufactured for Internal Use, net - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets manufactured for internal use
Cost	$ 94,719	$ 86,968
Accumulated depreciation	(58,551)	(56,128)
Net	36,168	30,840
Depreciation expense	2,200	1,800	$ 2,400
Assets Manufactured for Internal Use
Assets manufactured for internal use
Cost	40,667	44,521
Accumulated depreciation	(20,035)	(22,838)
Net	20,632	21,683
Depreciation expense	2,800	2,400	$ 2,500
Assets Manufactured for Internal Use | Internal use assets
Assets manufactured for internal use
Cost	40,366	42,776
Assets Manufactured for Internal Use | Construction in process.
Assets manufactured for internal use
Cost	$ 301	$ 1,745