Summary of Quarterly Financial Information (unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 199,309	$ 186,402	$ 178,529	$ 167,271	$ 182,297	$ 186,547	$ 176,349	$ 164,333	$ 178,807	$ 175,704	$ 165,785	$ 157,057	$ 731,511	$ 709,526	$ 677,353
Operating income	58,490	48,538	49,665	34,164	50,191	56,008	50,659	36,045	52,195	53,442	44,162	34,097	190,857	192,903	183,896
Net income	39,781	29,230	30,184	18,058	32,260	35,489	31,359	19,145	33,008	32,897	24,640	16,705	$ 117,253	$ 118,253	$ 107,250
Net income attributable to common stockholders	$ 22,842	$ 16,342	$ 16,651	$ 9,768	$ 17,272	$ 18,806	$ 16,584	$ 9,985	$ 16,966	$ 15,973	$ 11,089	$ 7,428
Basic and diluted net income per share attributable to common stockholders (USD per share)	$ 0.13	$ 0.10	$ 0.10	$ 0.06	$ 0.11	$ 0.12	$ 0.10	$ 0.06	$ 0.11	$ 0.12	$ 0.09	$ 0.06