Summary of Quarterly Financial Information (unaudited) (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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 183,070	$ 187,320	$ 177,124	$ 164,954	$ 179,263	$ 175,848	$ 165,815	$ 157,074	$ 165,200	$ 175,779	$ 164,773	$ 151,882	$ 712,468	$ 678,000	$ 657,634
Operating income	50,964	56,781	51,434	36,666	52,651	53,586	44,192	34,114	37,230	45,343	45,039	23,757	195,845	184,543	151,369
Net income	32,260	35,489	31,359	19,145	33,008	32,897	24,640	16,705	19,370	26,085	26,585	7,888	$ 118,253	$ 107,250	$ 79,928
Net income attributable to common stockholders	$ 17,272	$ 18,806	$ 16,584	$ 9,985	$ 16,966	$ 15,973	$ 11,089	$ 7,428	$ 8,252	$ 11,220	$ 11,120	$ 3,138
Basic and diluted net income per share attributable to common stockholders (USD per share)	$ 0.11	$ 0.12	$ 0.10	$ 0.06	$ 0.11	$ 0.12	$ 0.09	$ 0.06	$ 0.07	$ 0.10	$ 0.10	$ 0.03