Interim Financial Statements (Narrative) (Details) (USD $) Share data in Millions, unless otherwise specified	0 Months Ended	3 Months Ended			6 Months Ended				6 Months Ended		12 Months Ended			3 Months Ended	6 Months Ended							3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended
	Apr. 28, 2011	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2011	Apr. 30, 2011	Jun. 30, 2011 Subordinate Tax Increment Revenue Bonds [Member]		Dec. 31, 2010 Subordinate Tax Increment Revenue Bonds [Member]		Apr. 13, 2011 Palm Coast [Member]	Jun. 30, 2010 Sheridan [Member]	Jun. 30, 2010 Sheridan [Member]	Apr. 02, 2010 Sheridan [Member]	Jun. 30, 2011 Accumulated Other Comprehensive Loss [Member]	Dec. 31, 2010 Accumulated Other Comprehensive Loss [Member]	Jun. 30, 2011 Restricted Deposits And Mortgage Escrows [Member]	Dec. 31, 2010 Restricted Deposits And Mortgage Escrows [Member]	Jun. 30, 2011 Tenant Base [Member]	Jun. 30, 2011 Share Options [Member]	Jun. 30, 2010 Share Options [Member]	Jun. 30, 2011 Share Options [Member]	Jun. 30, 2010 Share Options [Member]	Jun. 30, 2011 Operating Partnership Units [Member]	Jun. 30, 2010 Operating Partnership Units [Member]	Jun. 30, 2011 Operating Partnership Units [Member]	Jun. 30, 2010 Operating Partnership Units [Member]	Jun. 30, 2011 Share Options And Awards [Member]	Jun. 30, 2010 Share Options And Awards [Member]
Date of establishment					1948
Square footage of operating properties					73,400,000
Percentage of rental revenues accounted from our largest tenant																					3.10%
Impairment losses related to incremental tax revenue bonds	$ 18,700,000								$ 18,737,000	[1]	$ 11,717,000	[1]
Impairment losses related to property		2,600,000	0		3,800,000	300,000
Restricted cash																			2,000,000	1,800,000
Escrow deposits																			30,000,000	8,400,000
Anti-dilutive securities excluded from the calculation of net income per common share - diluted																						2.4	3.6	2.4	3.6	1.6	1.7	1.6	1.7	1.0	0.8
Value of shares issued in exchange for interests in real estate joint ventures and partnerships					0	700,000
Accrued construction costs					5,300,000	5,100,000
Net cash paid for interest on debt					70,900,000	71,000,000
Cash paid for income taxes					1,600,000	2,000,000
Noncash or part noncash minority interest acquisition value of equity assumed					1,700,000	(900,000)
Ownership percentage in real estate properties sold during the period				80.00%
Number of properties sold				2
Impairment losses related to partially owned real estate joint ventures and partnerships		0	15,800,000		0	15,800,000								15,800,000	15,800,000
Ownership interest retained in unconsolidated investment				20.00%
Unconsolidated equity investment value				9,800,000
Debt assumed by unconsolidated joint venture				28,100,000
Accumulated comprehensive loss from derivative instruments																	10,700,000	11,700,000
Accumulated comprehensive loss from retirement liability																	10,100,000	10,100,000
Ownership percentage in joint ventures							47.80%	50.00%					50.00%			50.00%
Guaranty of debt service reduction					$ 22,900,000

[1] Total gains (losses) exclude impairments on disposed assets.