Supplemental Disclosure of Non-Cash Investing and Financing Activities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Supplemental Disclosure of Non-Cash Investing and Financing Activities
Issuance of common stock as deferred compensation	$ 722	$ 699
Issuance of units in the operating partnership	42,239	62,443
Redemption of units in the operating partnership	87,513	865
Derivative instruments at fair value	92	1,870
Assignment of debt investment to joint venture	25,362	286,571
Mortgage assigned to joint venture		30,000
Tenant improvements and capital expenditures payable	1,738	3,990
Fair value adjustment to noncontrolling interest in operating partnership	61,238	39,040
Accrued acquisition liabilities		34,500
Assumption of mortgage loans		943,767
Consolidation of real estate investments and other adjustments		1,156,929
Consolidation of real estate investments - noncontrolling interest in other partnerships		87,264
Repayment of mezzanine loans	13,750
Redemption of Series E units	31,698
Repayment of financing receivable	28,195
Investment in joint venture	5,135
Capital lease obligation	28,132
Deconsolidation of a subsidiary	104,107
Transfer to net assets held for sale	4,901
Transfer to liabilities related to net assets held for sale	136
Issuance of preferred units	$ 47,550