Supplementary Quarterly Financial Data (Unaudited) - Additional Information (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 281,842	$ 284,156	$ 276,003	$ 263,008	$ 261,016	$ 257,183	$ 244,273	$ 229,403	$ 1,105,009	$ 991,875	$ 782,270
Cost of operations	140,728	144,275	139,596	138,805	137,832	134,459	133,971	135,775	563,404	542,037	487,614
Revenues less cost of operations	141,114	139,881	136,407	124,203	123,184	122,724	110,302	93,628	541,605	449,838	294,656
Net income	229,315	101,075	94,098	89,734	90,416	127,226	90,040	89,407	514,222	397,089	209,536
Net income attributable to common stockholders	$ 215,983	$ 93,764	$ 87,006	$ 82,282	$ 82,403	$ 118,088	$ 83,044	$ 82,592	$ 479,013	$ 366,127	$ 189,474
Earnings per common share-basic (in dollars per share)	$ 1.71	$ 0.74	$ 0.69	$ 0.65	$ 0.65	$ 0.94	$ 0.66	$ 0.66	$ 3.79	$ 2.92	$ 1.58
Earnings per common share-diluted (in dollars per share)	$ 1.69	$ 0.74	$ 0.69	$ 0.64	$ 0.65	$ 0.93	$ 0.66	$ 0.66	$ 3.76	$ 2.91	$ 1.56