CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
OPERATING ACTIVITIES
Net income	$ 48,399,000	$ 48,473,000	$ 33,225,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization from continuing operations	73,290,000	70,314,000	65,789,000
Depreciation and amortization from discontinued operations	0	0	130,000
Stock-based compensation expense	6,733,000	5,146,000	4,229,000
Gain on sales of land and real estate investments	(3,026,000)	(9,286,000)	(822,000)
Changes in operating assets and liabilities:
Accrued income and other assets	(782,000)	523,000	(1,629,000)
Accounts payable, accrued expenses and prepaid rent	6,928,000	2,315,000	8,906,000
Other	(157,000)	(28,000)	(78,000)
NET CASH PROVIDED BY OPERATING ACTIVITIES	131,385,000	117,457,000	109,750,000
INVESTING ACTIVITIES
Real estate development	(95,032,000)	(97,696,000)	(76,240,000)
Purchases of real estate	(31,574,000)	(48,805,000)	(72,397,000)
Real estate improvements	(24,514,000)	(20,524,000)	(20,807,000)
Proceeds from sales of real estate investments	5,156,000	20,625,000	4,273,000
Capital contributions to unconsolidated investment	0	(5,132,000)	0
Repayments on mortgage loans receivable	116,000	3,902,000	463,000
Changes in accrued development costs	(1,705,000)	241,000	509,000
Changes in other assets and other liabilities	(8,865,000)	(12,181,000)	(11,912,000)
NET CASH USED IN INVESTING ACTIVITIES	(156,418,000)	(159,570,000)	(176,111,000)
FINANCING ACTIVITIES
Proceeds from unsecured bank credit facilities	420,104,000	350,214,000	424,375,000
Repayments on unsecured bank credit facilities	(368,669,000)	(339,765,000)	(411,583,000)
Repayments on secured debt	(102,337,000)	(48,846,000)	(107,953,000)
Proceeds from unsecured debt	150,000,000	75,000,000	175,000,000
Debt issuance costs	(1,952,000)	(499,000)	(2,222,000)
Distributions paid to stockholders (not including dividends accrued on unvested restricted stock)	(75,845,000)	(70,456,000)	(64,798,000)
Proceeds from common stock offerings	6,233,000	78,868,000	53,247,000
Proceeds from exercise of stock options	0	0	120,000
Proceeds from dividend reinvestment plan	256,000	216,000	206,000
Other	(2,720,000)	(2,616,000)	(1,281,000)
NET CASH PROVIDED BY FINANCING ACTIVITIES	25,070,000	42,116,000	65,111,000
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	37,000	3,000	(1,250,000)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	11,000	8,000	1,258,000
CASH AND CASH EQUIVALENTS AT END OF YEAR	48,000	11,000	8,000
SUPPLEMENTAL CASH FLOW INFORMATION
Cash paid for interest, net of amount capitalized of $5,257, $4,942, and $5,064 for 2015, 2014 and 2013, respectively	33,164,000	34,426,000	32,880,000
Fair value of debt assumed by the Company in the purchase of real estate	$ 0	$ 2,846,000	$ 0