Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information
Revenues	$ 37,393	$ 34,943	$ 32,387	$ 31,480	$ 30,755	$ 29,541	$ 29,227	$ 29,438	$ 136,203	$ 118,961	$ 104,974
Net income available to common stockholders	$ 17,840	$ 18,708	$ 16,984	$ 16,611	$ 20,043	$ 16,181	$ 17,338	$ 16,083	$ 70,143	$ 69,645	$ 54,159
Net income per common share available to common stockholders:
Basic (in dollars per share)	$ 0.49	$ 0.53	$ 0.48	$ 0.47	$ 0.58	$ 0.47	$ 0.50	$ 0.47	$ 1.97	$ 2.01	$ 1.64
Diluted (in dollars per share)	0.48	0.52	0.48	0.47	0.57	0.46	0.50	0.46	1.94	1.99	1.63
Dividends per share declared (in dollars per share)	0.54	0.51	0.51	0.51	0.51	0.51	0.51	0.51
Dividends declared and paid per common share (in dollars per share)	$ 0.54	$ 0.51	$ 0.51	$ 0.51	$ 0.51	$ 0.51	$ 0.51	$ 0.51	$ 2.070	$ 2.040	$ 1.905