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, 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 - $2.44, $2.52, and $2.72 per share in 2016, 2017 and 2018, 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 875,052, 1,370,457, and 1,706,474 shares of common stock, common stock offering, net of expenses in 2016, 2017 and 2018, respectively	59,283,000	0	59,283,000	0	0	0
Issuance of 3,326, 2,744, and 1,844 shares of common stock, dividend reinvestment plan in 2016, 2017 and 2018, respectively	228,000	0	228,000	0	0	0
Withheld 57,316, 33,695, and 23,824 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2016, 2017 and 2018, 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 - $2.44, $2.52, and $2.72 per share in 2016, 2017 and 2018, 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 875,052, 1,370,457, and 1,706,474 shares of common stock, common stock offering, net of expenses in 2016, 2017 and 2018, respectively	109,207,000	0	109,207,000	0	0	0
Issuance of 3,326, 2,744, and 1,844 shares of common stock, dividend reinvestment plan in 2016, 2017 and 2018, respectively	228,000	0	228,000	0	0	0
Withheld 57,316, 33,695, and 23,824 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2016, 2017 and 2018, respectively	(2,505,000)	0	(2,505,000)	0	0	0
Purchase of noncontrolling interest in joint venture	(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
Comprehensive income
Net income	88,636,000	0	0	88,506,000	0	130,000
Net unrealized change in fair value of interest rate swaps - cash flow hedges	1,353,000	0	0	0	1,353,000	0
Common dividends declared - $2.44, $2.52, and $2.72 per share in 2016, 2017 and 2018, respectively	(97,667,000)	0	0	(97,667,000)	0	0
Stock-based compensation, net of forfeitures	6,103,000	0	6,103,000	0	0	0
Issuance of 875,052, 1,370,457, and 1,706,474 shares of common stock, common stock offering, net of expenses in 2016, 2017 and 2018, respectively	157,319,000	1,000	157,318,000	0	0	0
Issuance of 3,326, 2,744, and 1,844 shares of common stock, dividend reinvestment plan in 2016, 2017 and 2018, respectively	164,000	0	164,000	0	0	0
Withheld 57,316, 33,695, and 23,824 shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock in 2016, 2017 and 2018, respectively	(2,055,000)	0	(2,055,000)	0	0	0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(136,000)	0	(136,000)	0	0	0
Distributions to noncontrolling interest	(194,000)	0	0	0	0	(194,000)
Contributions from noncontrolling interest	50,000	0	0	0	0	50,000
BALANCE at Dec. 31, 2018	$ 904,703,000	$ 4,000	$ 1,222,547,000	$ (326,193,000)	$ 6,701,000	$ 1,644,000