Assets Manufactured for Internal Use, net - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets manufactured for internal use
Cost	$ 86,968	$ 93,022
Accumulated depreciation	(56,128)	(62,991)
Net	30,840	30,031
Depreciation expense	1,800	2,400	$ 2,400
Assets Manufactured for Internal Use
Assets manufactured for internal use
Cost	44,521	32,229
Accumulated depreciation	(22,838)	(20,193)
Net	21,683	12,036
Depreciation expense	2,400	2,500	$ 2,200
Assets Manufactured for Internal Use | Internal use assets
Assets manufactured for internal use
Cost	42,776	31,759
Assets Manufactured for Internal Use | Construction in process.
Assets manufactured for internal use
Cost	$ 1,745	$ 470