Selected Quarterly Financial Data (Schedule of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Data [Abstract]
Total revenues	$ 355,972	$ 245,346	$ 308,530	$ 260,178	$ 291,750	$ 227,401	$ 252,613	$ 199,427	$ 294,830	$ 300,976	$ 275,667	$ 255,504	$ 1,170,026	$ 971,191	$ 1,126,977
Operating income	56,327	31,194	64,795	32,873	40,038	16,931	26,888	23,597	40,628	2,285	53,635	39,174	185,189	107,454	135,722
Net income	35,460	23,112	45,727	17,207	36,837	10,323	17,713	10,433	27,927	(5,000)	40,591	29,994	121,506	75,306	93,512
Net income (loss)	$ 35,486	$ 23,138	$ 45,717	$ 17,138	$ 36,754	$ 10,261	$ 17,635	$ 10,377	$ 27,853	$ (5,137)	$ 39,710	$ 29,406	$ 121,479	$ 75,027	$ 91,832
Basic, Per share amount	$ 0.33	$ 0.21	$ 0.42	$ 0.16	$ 0.34	$ 0.10	$ 0.16	$ 0.10	$ 0.26	$ (0.05)	$ 0.37	$ 0.28	$ 1.12	$ 0.70	$ 0.86
Diluted, Per share amount	$ 0.32	$ 0.21	$ 0.42	$ 0.16	$ 0.34	$ 0.09	$ 0.16	$ 0.10	$ 0.26	$ (0.05)	$ 0.37	$ 0.27	$ 1.11	$ 0.69	$ 0.85