Long-Term Debt (Details) - Revolving credit facility - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Interest incurred	$ 415	$ 449	$ 1,913
Capitalized interest	0	0	534
Interest paid (net of capitalized interest)	$ 181	$ 284	$ 1,169
Weighted average interest rate	0.00%	0.00%	2.20%