Interim Financial Statements (Narrative) (Details) (USD $)	3 Months Ended	9 Months Ended					9 Months Ended
	Mar. 31, 2010	Sep. 30, 2011	Sep. 30, 2010	Apr. 30, 2011	Sep. 30, 2011 Accumulated Other Comprehensive Loss [Member]	Dec. 31, 2010 Accumulated Other Comprehensive Loss [Member]	Sep. 30, 2011 Noncontrolling Interests [Member]	Sep. 30, 2010 Noncontrolling Interests [Member]	Sep. 30, 2011 Restricted Deposits And Mortgage Escrows [Member]	Dec. 31, 2010 Restricted Deposits And Mortgage Escrows [Member]	Sep. 30, 2011 Tenant Base [Member]
Interim Financial Statements [Line Items]
Date of establishment		1948
Square footage of operating properties		74,700,000
Percentage of rental revenues accounted from our largest tenant											3.20%
Restricted cash									$ 1,700,000	$ 1,800,000
Escrow deposits									9,300,000	8,400,000
Shares issued in exchange for noncontrolling interests							0	746,000
Accrued construction costs		11,500,000	6,000,000
Net cash paid for interest on debt		117,400,000	113,700,000
Cash paid for income taxes		1,600,000	2,100,000
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
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					10,600,000	11,700,000
Accumulated comprehensive loss from retirement liability					10,100,000	10,100,000
Ownership percentage in joint ventures		47.80%		50.00%
Guaranty of debt service reduction		$ 22,900,000