Supplemental Disclosure to Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Disclosure to Statements of Cash Flows [Abstract]
Write-off of fully depreciated property and deferred costs	$ 38,131	$ 22,269	$ 31,069
Write-off of depreciated property and deferred costs due to sale	173,083	202,695	106,698
Write-off of preferred units costs due to redemption	0	1,214	2,806
Assumption of mortgage loans in connection with the acquisition of properties	0	229,751	12,537
Increase in investments in and advances to unconsolidated joint ventures due to disposition/development activity	(11,948)	0	0
Changes in accrued development capital expenditures	6,264	17,053	2,297
Unrealized (loss) gain on cash flow hedge	$ (3,419)	$ 1,584	$ 0