Supplemental Disclosures of Cash Flow Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Supplemental Disclosures of Cash Flow Information [Abstract]
Interest paid			$ 96,400,000	$ 78,500,000
Interest capitalized to properties under development			317,000	5,000
Income taxes paid			783,000	947,000
Non-cash investing and financing activities [Abstract]
Share-based compensation expense	1,800,000	1,300,000	6,100,000	4,800,000
Receivable for the sale of investment property		799,000		799,000
Mortgage notes payable to third-party lenders	67,400,000		67,400,000
Note receivable assumed in connection with 2011 acquisition	8,780,000		8,780,000		0
Net premiums on mortgage notes payable to third-party lenders	957,000		957,000
Increase in buildings and improvements and accounts payable as a result of accrued costs on properties under development	$ 4,600,000	$ 545,000	$ 4,600,000	$ 545,000