Quarterly Financial Data (Summary of Quarterly Financial Data) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Data [Abstract]
Revenue	$ 69,300	$ 74,468	$ 64,312	$ 63,553	$ 71,234	$ 65,767	$ 68,564	$ 75,885	$ 271,633		$ 281,450		$ 340,120
Operating income	3,921	7,155	4,065	11,154	7,972	7,686	10,732	13,496
Net income (loss)	$ 21,357	$ 3,916	$ 389	$ 7,414	$ (3,827)	$ 4,322	$ 5,853	$ 6,162	$ 33,076	[1]	$ 12,510	[1]	$ 20,132	[1]
Basic income per common share	$ 0.69	$ 0.13	$ 0.01	$ 0.25	$ (0.13)	$ 0.14	$ 0.19	$ 0.21

[1] There is no material difference between net income and comprehensive income.