Supplemental Disclosures of Cash Flow Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Supplemental Disclosures of Cash Flow Information [Abstract]
Interest paid (in millions)			$ 49,000,000	$ 41,300,000
Interest capitalized to properties.			173,000	3,000
Income taxes paid			946,000	872,000
Share-based compensation expense (in millions)	2,200,000	1,700,000	4,300,000	3,500,000
Sale of an investment property		799,000		799,000
Receivable for the sale of excess land from a property held for sale		600,000		600,000
Receivable for the sale of excess land	95,000		95,000
Total receivable on property held for sale	695,000		695,000
Mortgage notes payable to third party lenders (in millions)	58,600,000		58,600,000
Net premiums	957,000		957,000
Buildings and improvements and accounts payable (in millions)	$ 3,600,000		$ 3,600,000