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, 2014	$ 575,615,000	$ 3,000	$ 874,335,000	$ (300,852,000)	$ (2,357,000)	$ 4,486,000
Comprehensive income
Net income	48,399,000	0	0	47,866,000	0	533,000
Net unrealized change in fair value of interest rate swaps - cash flow hedges	(1,099,000)	0	0	0	(1,099,000)	0
Common dividends declared - $2.34, $2.44 and $2.52 per share in 2015, 2016 and 2017, respectively	(75,906,000)	0	0	(75,906,000)	0	0
Stock-based compensation, net of forfeitures	8,423,000	0	8,423,000	0	0	0
Issuance of 106,751, 875,052 and 1,370,457 shares of common stock, common stock offering, net of expenses in 2015, 2016 and 2017, respectively	6,233,000	0	6,233,000	0	0	0
Issuance of 4,536, 3,326 and 2,744 shares of common stock, dividend reinvestment plan in 2015, 2016 and 2017, respectively	257,000	0	257,000	0	0	0
Withheld 32,409, 57,316 and 33,695 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2015, 2016 and 2017, respectively	(2,041,000)	0	(2,041,000)	0	0	0
Distributions to noncontrolling interest	(680,000)	0	0	0	0	(680,000)
BALANCE at Dec. 31, 2015	559,201,000	3,000	887,207,000	(328,892,000)	(3,456,000)	4,339,000
Comprehensive income
Net income	96,094,000	0	0	95,509,000	0	585,000
Net unrealized change in fair value of interest rate swaps - cash flow hedges	5,451,000	0	0	0	5,451,000	0
Common dividends declared - $2.34, $2.44 and $2.52 per share in 2015, 2016 and 2017, respectively	(80,272,000)	0	0	(80,272,000)	0	0
Stock-based compensation, net of forfeitures	5,831,000	0	5,831,000	0	0	0
Issuance of 106,751, 875,052 and 1,370,457 shares of common stock, common stock offering, net of expenses in 2015, 2016 and 2017, respectively	59,283,000	0	59,283,000	0	0	0
Issuance of 4,536, 3,326 and 2,744 shares of common stock, dividend reinvestment plan in 2015, 2016 and 2017, respectively	228,000	0	228,000	0	0	0
Withheld 32,409, 57,316 and 33,695 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2015, 2016 and 2017, respectively	(3,231,000)	0	(3,231,000)	0	0	0
Distributions to noncontrolling interest	(719,000)	0	0	0	0	(719,000)
BALANCE at Dec. 31, 2016	641,866,000	3,000	949,318,000	(313,655,000)	1,995,000	4,205,000
Comprehensive income
Net income	83,589,000	0	0	83,183,000	0	406,000
Net unrealized change in fair value of interest rate swaps - cash flow hedges	3,353,000	0	0	0	3,353,000	0
Common dividends declared - $2.34, $2.44 and $2.52 per share in 2015, 2016 and 2017, respectively	(86,560,000)	0	0	(86,560,000)	0	0
Stock-based compensation, net of forfeitures	7,012,000	0	7,012,000	0	0	0
Issuance of 106,751, 875,052 and 1,370,457 shares of common stock, common stock offering, net of expenses in 2015, 2016 and 2017, respectively	109,207,000	0	109,207,000	0	0	0
Issuance of 4,536, 3,326 and 2,744 shares of common stock, dividend reinvestment plan in 2015, 2016 and 2017, respectively	228,000	0	228,000	0	0	0
Withheld 32,409, 57,316 and 33,695 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2015, 2016 and 2017, respectively	(2,505,000)	0	(2,505,000)	0	0	0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(4,704,000)	0	(2,107,000)	0	0	(2,597,000)
Distributions to noncontrolling interest	(478,000)	0	0	0	0	(478,000)
Contributions from noncontrolling interest	122,000	0	0	0	0	122,000
BALANCE at Dec. 31, 2017	$ 751,130,000	$ 3,000	$ 1,061,153,000	$ (317,032,000)	$ 5,348,000	$ 1,658,000