CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
OPERATING ACTIVITIES
Net income	$ 96,094,000	$ 48,399,000	$ 48,473,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	77,935,000	73,290,000	70,314,000
Stock-based compensation expense	4,590,000	6,733,000	5,146,000
Gain, net of loss, on sales of real estate investments and non-operating real estate	(42,903,000)	(3,026,000)	(9,286,000)
Changes in operating assets and liabilities:
Accrued income and other assets	(2,883,000)	(2,118,000)	(713,000)
Accounts payable, accrued expenses and prepaid rent	5,736,000	6,928,000	2,315,000
Other	295,000	(157,000)	(28,000)
NET CASH PROVIDED BY OPERATING ACTIVITIES	138,864,000	130,049,000	116,221,000
INVESTING ACTIVITIES
Real estate development	(203,765,000)	(95,032,000)	(97,696,000)
Purchases of real estate	(27,668,000)	(31,574,000)	(48,805,000)
Real estate improvements	(23,671,000)	(24,514,000)	(20,524,000)
Net proceeds from sales of real estate investments and non-operating real estate	78,780,000	5,156,000	20,625,000
Capital contributions to unconsolidated investment	0	0	(5,132,000)
Repayments on mortgage loans receivable	123,000	116,000	3,902,000
Changes in accrued development costs	3,629,000	(1,705,000)	241,000
Changes in other assets and other liabilities	(13,900,000)	(8,865,000)	(12,181,000)
NET CASH USED IN INVESTING ACTIVITIES	(186,472,000)	(156,418,000)	(159,570,000)
FINANCING ACTIVITIES
Proceeds from unsecured bank credit facilities	608,349,000	420,104,000	350,214,000
Repayments on unsecured bank credit facilities	(567,165,000)	(368,669,000)	(339,765,000)
Proceeds from Issuance of Unsecured Debt	205,000,000	150,000,000	75,000,000
Repayments of Unsecured Debt	(80,000,000)	0	0
Repayments on secured debt	(92,773,000)	(102,337,000)	(48,846,000)
Debt issuance costs	(1,487,000)	(1,952,000)	(499,000)
Distributions paid to stockholders (not including dividends accrued on unvested restricted stock)	(80,899,000)	(75,845,000)	(70,456,000)
Proceeds from common stock offerings	59,283,000	6,233,000	78,868,000
Proceeds from dividend reinvestment plan	236,000	256,000	216,000
Other	(2,462,000)	(1,384,000)	(1,380,000)
NET CASH PROVIDED BY FINANCING ACTIVITIES	48,082,000	26,406,000	43,352,000
INCREASE IN CASH AND CASH EQUIVALENTS	474,000	37,000	3,000
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	48,000	11,000	8,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	522,000	48,000	11,000
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid for interest, net of amount capitalized of $5,340, $5,257, and $4,942 for 2016, 2015 and 2014, respectively	33,595,000	33,164,000	34,426,000
Fair value of debt assumed by the Company in the purchase of real estate	$ 0	$ 0	$ 2,847,000