CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES
Net income	$ 108,391,000	$ 123,340,000	$ 88,636,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	116,359,000	104,724,000	91,704,000
Stock-based compensation expense	6,579,000	6,838,000	5,283,000
Gain on sales of real estate investments and non-operating real estate	(13,145,000)	(41,151,000)	(14,359,000)
Gain on casualties and involuntary conversion on real estate assets	(161,000)	(180,000)	(1,245,000)
Changes in operating assets and liabilities:
Accrued income and other assets	(4,615,000)	(5,558,000)	(4,091,000)
Accounts payable, accrued expenses and prepaid rent	(18,851,000)	6,514,000	(2,682,000)
Other	1,728,000	1,385,000	1,485,000
NET CASH PROVIDED BY OPERATING ACTIVITIES	196,285,000	195,912,000	164,731,000
INVESTING ACTIVITIES
Development and value-add properties	(195,446,000)	(318,288,000)	(167,667,000)
Purchases of real estate	(49,199,000)	(142,712,000)	(57,152,000)
Real estate improvements	(33,131,000)	(37,775,000)	(37,502,000)
Net proceeds from sales of real estate investments and non-operating real estate	21,565,000	66,737,000	24,508,000
Proceeds from casualties and involuntary conversion on real estate assets	242,000	723,000	(1,635,000)
Repayments on mortgage loans receivable	1,679,000	915,000	1,987,000
Changes in accrued development costs	(5,339,000)	(3,644,000)	5,711,000
Changes in other assets and other liabilities	(28,627,000)	(9,293,000)	(12,955,000)
NET CASH USED IN INVESTING ACTIVITIES	(288,256,000)	(443,337,000)	(241,435,000)
FINANCING ACTIVITIES
Proceeds from unsecured bank credit facilities	625,387,000	932,658,000	448,100,000
Repayments on unsecured bank credit facilities	(613,097,000)	(1,015,678,000)	(448,709,000)
Proceeds from unsecured debt	275,000,000	290,000,000	60,000,000
Repayments on unsecured debt	(105,000,000)	(75,000,000)	50,000,000
Repayments on secured debt	(54,306,000)	(55,593,000)	(11,289,000)
Debt issuance costs	(1,090,000)	(893,000)	(1,922,000)
Distributions paid to stockholders (not including dividends accrued)	(119,765,000)	(108,795,000)	(71,294,000)
Proceeds from common stock offerings	90,721,000	284,710,000	157,319,000
Proceeds from dividend reinvestment plan	0	212,000	221,000
Other	(6,082,000)	(4,346,000)	(5,364,000)
NET CASH PROVIDED BY FINANCING ACTIVITIES	91,768,000	247,275,000	77,062,000
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(203,000)	(150,000)	358,000
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	224,000	374,000	16,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	21,000	224,000	374,000
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid for interest, net of amount capitalized of $9,651, $8,453, and $6,334 for 2020, 2019, and 2018, respectively	32,362,000	30,839,000	33,458,000
Cash paid for operating lease liability	1,476,000	1,314,000	0
NON-CASH OPERATING ACTIVITY
Operating lease liabilities arising from obtaining right of use assets	$ 495,000	$ 15,435,000	$ 0