Summarized Quarterly Financial Data (Unaudited) (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
Selected Quarterly Financial Information [Abstract]
Revenues	$ 118,707	$ 108,512	$ 112,720	$ 114,912	$ 119,279	$ 102,156	$ 108,487	$ 111,166	$ 454,851	$ 441,088	$ 434,437
Income from operations	11,688	8,224	13,854	18,443	19,654	4,830	12,896	20,092	52,209	57,472	69,698
Net income	$ (6,495)	$ 6,227	$ 10,302	$ 10,578	$ 11,718	$ 2,878	$ 7,786	$ 12,420	$ 20,612	$ 34,802	$ 40,023
Net income per share:
Basic	$ (0.61)	$ 0.58	$ 0.96	$ 1.00	$ 1.10	$ 0.27	$ 0.73	$ 1.17	$ 1.93	$ 3.28	$ 3.78
Diluted	$ (0.61)	$ 0.56	$ 0.92	$ 0.95	$ 1.07	$ 0.27	$ 0.72	$ 1.15	$ 1.84	$ 3.21	$ 3.73