Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Information
Revenues (in dollars)	$ 22,298	$ 21,429	$ 21,180	$ 20,253	$ 19,820	$ 18,359	$ 17,925	$ 17,592	$ 85,160	$ 73,696	$ 68,794
Net income from discontinued operations (in dollars)	(32)	(30)	(74)	(94)	256	25	(43)	(45)	(230)	193	(113)
Net income available to common stockholders (in dollars)	$ 11,656	$ 11,472	$ 11,311	$ 5,393	$ 9,583	$ 5,571	$ 7,739	$ 6,694	$ 39,832	$ 29,587	$ 29,410
Net income per common share from continuing operations allocable to common stockholders:
Basic (in dollars per share)	$ 0.39	$ 0.38	$ 0.38	$ 0.21	$ 0.36	$ 0.22	$ 0.33	$ 0.29	$ 1.37	$ 1.20	$ 1.28
Diluted (in dollars per share)	$ 0.39	$ 0.38	$ 0.38	$ 0.21	$ 0.36	$ 0.22	$ 0.33	$ 0.29	$ 1.37	$ 1.20	$ 1.28
Net (loss) income per common share from discontinued operations:
Basic (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.01	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.01
Diluted (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.01	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ 0.01
Net income per common share available to common stockholders:
Basic (in dollars per share)	$ 0.39	$ 0.38	$ 0.38	$ 0.20	$ 0.37	$ 0.22	$ 0.33	$ 0.29	$ 1.36	$ 1.21	$ 1.27
Diluted (in dollars per share)	$ 0.39	$ 0.38	$ 0.37	$ 0.20	$ 0.37	$ 0.22	$ 0.33	$ 0.29	$ 1.36	$ 1.21	$ 1.27
Dividends per share declared (in dollars per share)	$ 0.42	$ 0.42	$ 0.42	$ 0.42	$ 0.41	$ 0.39	$ 0.39	$ 0.39
Dividend per share paid(in dollars per share)	$ 0.42	$ 0.42	$ 0.42	$ 0.42	$ 0.41	$ 0.39	$ 0.39	$ 0.39	$ 1.68	$ 1.58	$ 1.56