Selected Quarterly Financial Data (Schedule of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	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, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected quarterly financial data
Total revenue	$ 383,413	$ 331,542	$ 389,050	$ 273,255	$ 332,194	$ 289,796	$ 393,222	$ 303,429	$ 355,972	$ 245,346	$ 308,530	$ 260,178	$ 1,377,260	$ 1,318,641	$ 1,170,026
Operating income	72,892	67,384	88,802	34,082	71,484	40,160	77,970	33,588	56,327	31,194	64,795	32,873	263,160	223,202	185,189
Net income (loss)	48,919	45,490	53,120	22,829	51,565	25,272	54,131	18,172	35,460	23,112	45,727	17,207	170,358	149,140	121,506
Net income (loss)	$ 48,857	$ 45,387	$ 53,042	$ 22,809	$ 51,573	$ 25,266	$ 54,036	$ 18,164	$ 35,486	$ 23,138	$ 45,717	$ 17,138	$ 170,095	$ 149,039	$ 121,479
Basic, Per share amount	$ 0.45	$ 0.42	$ 0.49	$ 0.21	$ 0.47	$ 0.23	$ 0.50	$ 0.17	$ 0.33	$ 0.21	$ 0.42	$ 0.16	$ 1.56	$ 1.37	$ 1.12
Diluted, Per share amount	$ 0.44	$ 0.41	$ 0.49	$ 0.21	$ 0.47	$ 0.23	$ 0.49	$ 0.17	$ 0.32	$ 0.21	$ 0.42	$ 0.16	$ 1.54	$ 1.36	$ 1.11