Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Cash paid for interest	$ 22	$ 2	$ 13
Cash paid for federal and state taxes	459	2,589	6,197
Non-cash transactions:
Change in accounts payable related to property and equipment additions	70	679	1,707
Property and equipment acquired under capital leases	$ 1,808	$ 0	$ 0