Supplemental Disclosure of Non-Cash Investing and Financing Activities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Supplemental Disclosure of Non-Cash Investing and Financing Activities
Issuance of common stock as deferred compensation	$ 544	$ 545
Issuance of units in the operating partnership	35,917	20,222
Redemption of units in the operating partnership	483	725
Derivative instruments at fair value	314	1,071
Assignment of debt investment to joint venture		286,571
Mortgage assigned upon asset sale	59,099	30,000
Tenant improvements and capital expenditures payable	12,294	8,389
Assumption of mortgage loans		663,767
Fair value adjustment to noncontrolling interest in operating partnership	46,810	42,296
Accrued acquisition liabilities		44,500
Consolidation of real estate investments		557,314
Transfer to net assets held for sale	86,339
Transfer to liabilities related to net assets held for sale	62,792
Repayment of mezzanine loan	3,750
Redemption of Series E units	31,968
Repayment of financing receivables	$ 28,195