Quarterly Financial Information (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, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information
Revenues	$ 43,587	$ 41,776	$ 41,472	$ 41,810	$ 41,729	$ 41,246	$ 42,468	$ 42,622	$ 168,645	$ 168,065	$ 161,583
Net income available to common stockholders	$ 30,645	$ 34,782	$ 68,658	$ 20,271	$ 19,753	$ 20,536	$ 25,273	$ 21,416	$ 154,356	$ 86,978	$ 84,730
Net income per common share available to common stockholders:
Basic (in dollars per share)	$ 0.78	$ 0.88	$ 1.74	$ 0.51	$ 0.50	$ 0.52	$ 0.64	$ 0.54	$ 3.91	$ 2.21	$ 2.21
Diluted (in dollars per share)	0.77	0.88	1.73	0.51	0.50	0.52	0.64	0.54	3.89	2.20	2.21
Dividends declared and paid per common share (in dollars per share)	0.57	0.57	0.57	0.57	0.57	0.57	0.57	0.57
Dividends declared and paid per common share (in dollars per share)	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 2.280	$ 2.280	$ 2.190