CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income	$ 83,589,000	$ 96,094,000	$ 48,399,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	83,874,000	77,935,000	73,290,000
Stock-based compensation expense	5,521,000	4,590,000	6,733,000
Gain, net of loss, on sales of real estate investments and non-operating real estate	(22,148,000)	(42,903,000)	(3,026,000)
Changes in operating assets and liabilities:
Accrued income and other assets	(5,034,000)	(2,883,000)	(2,118,000)
Accounts payable, accrued expenses and prepaid rent	8,333,000	5,736,000	6,928,000
Other	879,000	295,000	(157,000)
NET CASH PROVIDED BY OPERATING ACTIVITIES	155,014,000	138,864,000	130,049,000
INVESTING ACTIVITIES
Real estate development	(124,938,000)	(203,765,000)	(95,032,000)
Purchases of real estate	(55,195,000)	(27,668,000)	(31,574,000)
Real estate improvements	(26,158,000)	(23,778,000)	(25,062,000)
Net proceeds from sales of real estate investments and non-operating real estate	42,710,000	78,780,000	5,156,000
Repayments on mortgage loans receivable	171,000	123,000	116,000
Changes in accrued development costs	(144,000)	3,629,000	(1,705,000)
Changes in other assets and other liabilities	(15,872,000)	(13,793,000)	(8,317,000)
NET CASH USED IN INVESTING ACTIVITIES	(179,426,000)	(186,472,000)	(156,418,000)
FINANCING ACTIVITIES
Proceeds from unsecured bank credit facilities	391,617,000	608,349,000	420,104,000
Repayments on unsecured bank credit facilities	(387,298,000)	(567,165,000)	(368,669,000)
Proceeds from Unsecured Debt	60,000,000	205,000,000	150,000,000
Repayments on Unsecured Debt	0	(80,000,000)	0
Repayments on secured debt	(58,209,000)	(92,773,000)	(102,337,000)
Debt issuance costs	(380,000)	(1,487,000)	(1,952,000)
Distributions paid to stockholders (not including dividends accrued on unvested restricted stock)	(86,725,000)	(80,899,000)	(75,845,000)
Proceeds from common stock offerings	109,207,000	59,283,000	6,233,000
Proceeds from dividend reinvestment plan	228,000	236,000	256,000
Other	(4,534,000)	(2,462,000)	(1,384,000)
NET CASH PROVIDED BY FINANCING ACTIVITIES	23,906,000	48,082,000	26,406,000
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(506,000)	474,000	37,000
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	522,000	48,000	11,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	16,000	522,000	48,000
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid for interest, net of amount capitalized of $5,765, $5,340, and $5,257 for 2017, 2016 and 2015, respectively	$ 33,634,000	$ 33,595,000	$ 33,164,000