Summary of Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 179,263	$ 175,848	$ 165,815	$ 157,074	$ 165,200	$ 175,779	$ 164,773	$ 151,882	$ 170,411	$ 169,441	$ 155,168	$ 140,306	$ 678,000	$ 657,634	$ 635,326
Operating income	52,651	53,586	44,192	34,114	37,230	45,343	45,039	23,757	31,282	43,412	42,539	24,088	184,543	151,369	141,321
Net income	33,008	32,897	24,640	16,705	19,370	26,085	26,585	7,888	10,964	22,734	25,281	11,231	$ 107,250	$ 79,928	$ 70,210
Net income attributable to common stockholders	$ 16,966	$ 15,973	$ 11,089	$ 7,428	$ 8,252	$ 11,220	$ 11,120	$ 3,138	$ 4,142	$ 8,322	$ 9,834	$ 4,369
Basic and diluted net income per share attributable to common stockholders (USD per share)	$ 0.11	$ 0.12	$ 0.09	$ 0.06	$ 0.07	$ 0.10	$ 0.10	$ 0.03	$ 0.04	$ 0.09	$ 0.10	$ 0.05