CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net income	$ 88,636,000	$ 83,589,000	$ 96,094,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	91,704,000	83,874,000	77,935,000
Stock-based compensation expense	5,283,000	5,521,000	4,590,000
Net gain on sales of real estate investments and non-operating real estate	(14,359,000)	(22,148,000)	(42,903,000)
Gain on casualties and involuntary conversion	(1,245,000)	0	0
Changes in operating assets and liabilities:
Accrued income and other assets	(4,091,000)	(5,034,000)	(2,883,000)
Accounts payable, accrued expenses and prepaid rent	(2,682,000)	8,333,000	5,736,000
Other	1,485,000	879,000	295,000
NET CASH PROVIDED BY OPERATING ACTIVITIES	164,731,000	155,014,000	138,864,000
INVESTING ACTIVITIES
Development and value-add properties	(167,667,000)	(124,938,000)	(203,765,000)
Purchases of real estate	(57,152,000)	(55,195,000)	(27,668,000)
Real estate improvements	(37,502,000)	(27,385,000)	(23,809,000)
Net proceeds from sales of real estate investments and non-operating real estate	24,508,000	42,710,000	78,780,000
Proceeds from casualties and involuntary conversion	1,635,000	0	0
Repayments on mortgage loans receivable	1,987,000	171,000	123,000
Changes in accrued development costs	5,711,000	(144,000)	3,629,000
Changes in other assets and other liabilities	(12,955,000)	(14,645,000)	(13,762,000)
NET CASH USED IN INVESTING ACTIVITIES	(241,435,000)	(179,426,000)	(186,472,000)
FINANCING ACTIVITIES
Proceeds from unsecured bank credit facilities	448,100,000	391,617,000	608,349,000
Repayments on unsecured bank credit facilities	(448,709,000)	(387,298,000)	(567,165,000)
Proceeds from Unsecured Debt	60,000,000	60,000,000	205,000,000
Repayments on Unsecured Debt	(50,000,000)	0	(80,000,000)
Repayments on secured debt	(11,289,000)	(58,209,000)	(92,773,000)
Debt issuance costs	(1,922,000)	(380,000)	(1,487,000)
Distributions paid to stockholders (not including dividends accrued)	(71,294,000)	(86,725,000)	(80,899,000)
Proceeds from common stock offerings	157,319,000	109,207,000	59,283,000
Proceeds from dividend reinvestment plan	221,000	228,000	236,000
Other	(5,364,000)	(4,534,000)	(2,462,000)
NET CASH PROVIDED BY FINANCING ACTIVITIES	77,062,000	23,906,000	48,082,000
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	358,000	(506,000)	474,000
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	16,000	522,000	48,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	374,000	16,000	522,000
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid for interest, net of amount capitalized of $6,334, $5,765, and $5,340 for 2018, 2017 and 2016, respectively	$ 33,458,000	$ 33,634,000	$ 33,595,000