Assets Manufactured for Internal Use, net - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets manufactured for internal use
Cost	$ 93,022	$ 87,497
Accumulated depreciation	(62,991)	(57,033)
Net	30,031	30,464
Depreciation expense	2,400	2,400	$ 3,200
Assets Manufactured for Internal Use
Assets manufactured for internal use
Cost	32,229	27,774
Accumulated depreciation	(20,193)	(17,927)
Net	12,036	9,847
Depreciation expense	2,500	2,200	$ 1,800
Assets Manufactured for Internal Use | Internal use assets
Assets manufactured for internal use
Cost	31,759	24,309
Assets Manufactured for Internal Use | Construction in process.
Assets manufactured for internal use
Cost	$ 470	$ 3,465