Quarterly Financial 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
Revenues	$ 41,729	$ 41,246	$ 42,468	$ 42,622	$ 42,141	$ 40,842	$ 39,996	$ 38,604	$ 168,065	$ 161,583	$ 136,203
Net income available to common stockholders	$ 19,753	$ 20,536	$ 25,273	$ 21,416	$ 20,577	$ 22,321	$ 22,075	$ 19,757	$ 86,978	$ 84,730	$ 70,143
Net income per common share available to common stockholders:
Basic (in dollars per share)	$ 0.50	$ 0.52	$ 0.64	$ 0.54	$ 0.53	$ 0.57	$ 0.58	$ 0.53	$ 2.21	$ 2.21	$ 1.97
Diluted (in dollars per share)	0.50	0.52	0.64	0.54	0.53	0.57	0.58	0.53	2.20	2.21	1.94
Dividends per share declared (in dollars per share)	0.57	0.57	0.57	0.57	0.57	0.54	0.54	0.54
Dividends declared and paid per common share (in dollars per share)	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.54	$ 0.54	$ 0.54	$ 2.28	$ 2.19	$ 2.07