Supplemental Disclosures of Cash Flow Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 item	Sep. 30, 2011 item	Dec. 31, 2011
Supplemental Disclosures of Cash Flow Information
Interest paid			$ 105,700,000	$ 96,400,000
Interest capitalized to properties under development			388,000	317,000
Income taxes paid			961,000	783,000
Non-cash investing and financing activities
Share-based compensation expense	2,200,000	1,800,000	7,800,000	6,100,000
Increase in buildings and improvements and accounts payable				4,600,000
Number of acquired properties with assumed mortgages			5	4
Mortgages assumed			70,000,000	67,400,000
Net premiums recorded on mortgages assumed			7,100,000	820,000
Notes receivable assumed					$ 8,800,000