Commitments (Schedule of Capital Leased Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Capital Leased Assets [Line Items]
Cost	$ 15,192	$ 9,100
Accumulated depreciation	(4,500)	(5,132)
Net book value	10,692	3,968
Computer Software and Equipment [Member]
Capital Leased Assets [Line Items]
Cost	8,699	8,511
Accumulated depreciation	(3,604)	(4,990)
Net book value	5,095	3,521
Yard and Auto Equipment [Member]
Capital Leased Assets [Line Items]
Cost	6,493	589
Accumulated depreciation	(896)	(142)
Net book value	$ 5,597	$ 447