Supplementary Quarterly Financial Data (Unaudited) (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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 335,801	$ 337,505	$ 323,602	$ 311,546	$ 307,353	$ 306,953	$ 296,813	$ 285,485	$ 1,308,454	$ 1,196,604	$ 1,105,009
Cost of operations	171,156	174,867	165,609	163,069	150,676	153,362	152,097	151,573	674,701	607,708	563,404
Income from operations	164,645	162,638	157,993	148,477	156,677	153,591	144,716	133,912	634,958	619,703	654,394
Net income	120,279	115,995	112,689	102,160	109,250	139,687	102,713	95,430	451,123	447,080	514,222
Net income attributable to common stockholders	$ 112,282	$ 108,087	$ 104,828	$ 94,770	$ 101,462	$ 130,418	$ 95,153	$ 88,256	$ 419,967	$ 415,289	$ 479,013
Earnings per common share-basic (in dollars per share)	$ 0.87	$ 0.84	$ 0.82	$ 0.74	$ 0.80	$ 1.03	$ 0.75	$ 0.70	$ 3.27	$ 3.29	$ 3.79
Earnings per common share-diluted (in dollars per share)	$ 0.86	$ 0.83	$ 0.81	$ 0.74	$ 0.80	$ 1.02	$ 0.75	$ 0.70	$ 3.24	$ 3.27	$ 3.76