Assets Manufactured for Internal Use, net - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets manufactured for internal use
Cost	$ 102,252	$ 94,719
Accumulated depreciation	(61,103)	(58,551)
Net	41,149	36,168
Depreciation expense	$ 3,200	2,200	$ 1,800
Assets Manufactured for Internal Use
Assets manufactured for internal use
Useful life	10 years
Cost	$ 49,118	40,667
Accumulated depreciation	(19,285)	(20,035)
Net	29,833	20,632
Depreciation expense	2,600	2,800		$ 2,400
Assets Manufactured for Internal Use | Internal use assets
Assets manufactured for internal use
Cost	47,509	40,366
Assets Manufactured for Internal Use | Construction in process.
Assets manufactured for internal use
Cost	$ 1,609	$ 301