Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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	$ 109,847	$ 104,965	$ 101,792	$ 94,226	$ 93,906	$ 98,331	$ 94,410	$ 94,464	$ 410,830	$ 381,111
Operating income (loss)	46,462	40,133	38,104	35,435	34,309	39,223	36,287	37,219	160,134	147,038	$ 154,746
Net income attributable to common stockholders	$ 36,728	$ 34,017	$ 31,333	$ 32,543	$ 25,561	$ 30,790	$ 29,959	$ 27,586	$ 134,621	$ 113,896	$ 91,939
Earnings per share attributable to common stockholders:
Basic (in dollars per share)	$ 0.78	$ 0.72	$ 0.66	$ 0.69	$ 0.55	$ 0.66	$ 0.64	$ 0.59	$ 2.85	$ 2.43	$ 1.98
Diluted (in dollars per share)	$ 0.75	$ 0.70	$ 0.65	$ 0.68	$ 0.54	$ 0.65	$ 0.63	$ 0.59	$ 2.79	$ 2.40	$ 1.96
Weighted-average shares outstanding:
Basic (shares)	47,324	47,316	47,304	47,146	46,842	46,830	46,819	46,683	47,273	46,794	46,353
Diluted (shares)	48,703	48,412	48,175	47,642	47,562	47,524	47,311	47,152	48,297	47,381	46,979