Selected Quarterly Financial Data (Schedule of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected quarterly financial data
Revenues	$ 332,194	$ 289,796	$ 393,222	$ 303,429	$ 355,972	$ 245,346	$ 308,530	$ 260,178	$ 291,750	$ 227,401	$ 252,613	$ 199,427	$ 1,318,641	$ 1,170,026	$ 971,191
Operating income	71,484	40,160	77,970	33,588	56,327	31,194	64,795	32,873	40,038	16,931	26,888	23,597	223,202	185,189	107,454
Net income (loss)	51,565	25,272	54,131	18,172	35,460	23,112	45,727	17,207	36,837	10,323	17,713	10,433	149,140	121,506	75,306
Net income (loss)	$ 51,573	$ 25,266	$ 54,036	$ 18,164	$ 35,486	$ 23,138	$ 45,717	$ 17,138	$ 36,754	$ 10,261	$ 17,635	$ 10,377	$ 149,039	$ 121,479	$ 75,027
Basic, Per share amount	$ 0.47	$ 0.23	$ 0.50	$ 0.17	$ 0.33	$ 0.21	$ 0.42	$ 0.16	$ 0.34	$ 0.10	$ 0.16	$ 0.10	$ 1.37	$ 1.12	$ 0.70
Diluted, Per share amount	$ 0.47	$ 0.23	$ 0.49	$ 0.17	$ 0.32	$ 0.21	$ 0.42	$ 0.16	$ 0.34	$ 0.09	$ 0.16	$ 0.10	$ 1.36	$ 1.11	$ 0.69