CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $)	Total	Common Stock	Additional Paid-in Capital	Distributions In Excess Of Earnings	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest in Joint Ventures
BALANCE at Dec. 31, 2011	$ 405,609,000	$ 3,000	$ 619,386,000	$ (216,560,000)	$ 0	$ 2,780,000
Comprehensive income
Net income	32,887,000	0	0	32,384,000	0	503,000
Net unrealized change in fair value of interest rate swaps	(392,000)	0	0	0	(392,000)	0
Common dividends declared - $2.10, $2.14 and $2.22 per share in 2012, 2013 and 2014, respectively	(61,073,000)	0	0	(61,073,000)	0	0
Stock-based compensation, net of forfeitures	4,447,000	0	4,447,000	0	0	0
Issuance of 2,179,153, 890,085 and 1,246,400 shares of common stock, common stock offering, net of expenses in 2012, 2013 and 2014, respectively	109,588,000	0	109,588,000	0	0	0
Issuance of 4,500 and 4,500 shares of common stock, options exercised in 2012 and 2013, respectively	108,000	0	108,000	0	0	0
Issuance of 3,915, 3,577 and 3,626 shares of common stock, dividend reinvestment plan in 2012, 2013 and 2014, respectively	205,000	0	205,000	0	0	0
Withheld 36,195, 9,412 and 31,673 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2012, 2013 and 2014, respectively	(1,784,000)	0	(1,784,000)	0	0	0
Distributions to noncontrolling interest	(537,000)	0	0	0	0	(537,000)
Proceeds from Noncontrolling Interests	2,118,000	0	0	0	0	2,118,000
BALANCE at Dec. 31, 2012	491,176,000	3,000	731,950,000	(245,249,000)	(392,000)	4,864,000
Comprehensive income
Net income	33,225,000	0	0	32,615,000	0	610,000
Net unrealized change in fair value of interest rate swaps	2,021,000	0	0	0	2,021,000	0
Common dividends declared - $2.10, $2.14 and $2.22 per share in 2012, 2013 and 2014, respectively	(65,535,000)	0	0	(65,535,000)	0	0
Stock-based compensation, net of forfeitures	5,540,000	0	5,540,000	0	0	0
Issuance of 2,179,153, 890,085 and 1,246,400 shares of common stock, common stock offering, net of expenses in 2012, 2013 and 2014, respectively	53,247,000	0	53,247,000	0	0	0
Issuance of 4,500 and 4,500 shares of common stock, options exercised in 2012 and 2013, respectively	120,000	0	120,000	0	0	0
Issuance of 3,915, 3,577 and 3,626 shares of common stock, dividend reinvestment plan in 2012, 2013 and 2014, respectively	206,000	0	206,000	0	0	0
Withheld 36,195, 9,412 and 31,673 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2012, 2013 and 2014, respectively	(528,000)	0	(528,000)	0	0	0
Distributions to noncontrolling interest	(767,000)	0	0	0	0	(767,000)
BALANCE at Dec. 31, 2013	518,705,000	3,000	790,535,000	(278,169,000)	1,629,000	4,707,000
Comprehensive income
Net income	48,473,000	0	0	47,941,000	0	532,000
Net unrealized change in fair value of interest rate swaps	(3,986,000)	0	0	0	(3,986,000)	0
Common dividends declared - $2.10, $2.14 and $2.22 per share in 2012, 2013 and 2014, respectively	(70,624,000)	0	0	(70,624,000)	0	0
Stock-based compensation, net of forfeitures	6,567,000	0	6,567,000	0	0	0
Issuance of 2,179,153, 890,085 and 1,246,400 shares of common stock, common stock offering, net of expenses in 2012, 2013 and 2014, respectively	78,868,000	0	78,868,000	0	0	0
Issuance of 3,915, 3,577 and 3,626 shares of common stock, dividend reinvestment plan in 2012, 2013 and 2014, respectively	227,000	0	227,000	0	0	0
Withheld 36,195, 9,412 and 31,673 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2012, 2013 and 2014, respectively	(1,862,000)	0	(1,862,000)	0	0	0
Distributions to noncontrolling interest	(753,000)	0	0	0	0	(753,000)
BALANCE at Dec. 31, 2014	$ 575,615,000	$ 3,000	$ 874,335,000	$ (300,852,000)	$ (2,357,000)	$ 4,486,000