Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Data (Unaudited)
Rental income	$ 58,271	$ 56,401	$ 55,934	$ 56,304	$ 54,629	$ 52,426	$ 48,584	$ 48,061	$ 226,910	$ 203,700	$ 168,074
Net income	12,724	1,966	15,204	24,726	13,191	11,756	11,954	13,059	54,620	49,960	45,997
Per Share data:
Net income per common share (in dollars per share)	$ 0.23	$ 0.04	$ 0.28	$ 0.45	$ 0.25	$ 0.25	$ 0.25	$ 0.28	$ 1.00	$ 1.03	$ 1.06
Common distributions declared (in dollars per share)	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.42	$ 0.42	$ 0.42	$ 1.72	$ 1.69	$ 1.67
Selected Quarterly Financial Data for Revenue Restated
Rental income	58,271	56,401	55,934	56,304	54,629	52,426	48,584	48,061	226,910	203,700	168,074
As previously reported
Selected Quarterly Financial Data (Unaudited)
Rental income	56,265	54,083	50,273	50,455
Selected Quarterly Financial Data for Revenue Restated
Rental income	56,265	54,083	50,273	50,455
Reclassifications | Discontinued operations
Selected Quarterly Financial Data (Unaudited)
Rental income	(1,636)	(1,657)	(1,689)	(2,394)
Selected Quarterly Financial Data for Revenue Restated
Rental income	$ (1,636)	$ (1,657)	$ (1,689)	$ (2,394)