Supplemental Disclosures of Cash Flow Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011 item
Supplemental Disclosures of Cash Flow Information
Interest paid			$ 55,700,000	$ 49,000,000
Interest capitalized to properties under development			277,000	173,000
Income taxes paid			818,000	946,000
Non-cash investing and financing activities
Share-based compensation expense	2,600,000	2,200,000	5,600,000	4,300,000
Increase in buildings and improvements and accounts payable			1,900,000	3,600,000
Loans receivable to fund development of real estate			889,000
Number of acquired properties with assumed mortgages				3
Mortgages assumed				58,600,000
Net premiums recorded on mortgages assumed				$ 820,000