Supplemental Disclosures of Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 property lease	Dec. 31, 2011 property	Dec. 31, 2010
Supplemental Disclosures of Cash Flow Information
Interest paid	$ 112,500,000	$ 102,000,000	$ 82,600,000
Interest capitalized to properties under development	498,000	438,000	10,000
Income taxes paid	1,000,000	871,000	907,000
Non-cash investing and financing activities
Share-based compensation expense	10,001,000	7,873,000	6,166,000
Number of acquired properties with assumed mortgages	8	4
Mortgages assumed	110,500,000	67,400,000
Net premiums recorded on mortgages assumed	10,000,000	820,000
Notes receivable assumed		8,800,000
Receivable recorded for the sale of an investment property as a result of an eminent domain action		799,000	799,000
Number of land leases with increases recorded to estimated legal obligations related to asset retirement obligations	2
Increases to estimated legal obligations related to asset retirement obligations	30,553	152,277	81,593
Increase in buildings and improvements and accounts payable	3,800,000	3,700,000
Receivable for the sale of excess land			$ 600,000