Significant Transactions - Asset Management Business Disposition (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Significant Acquisitions and Disposals [Line Items]
Revenues	$ 242,178	[1]	$ 246,843	[1]	$ 246,418	[1]	$ 248,924	[1]	$ 250,862	[1]	$ 243,943	[1]	$ 241,744	[1]	$ 237,504	[1]	$ 984,363	$ 974,053	$ 958,511
Expenses	178,370	[1]	179,376	[1]	180,621	[1]	183,646	[1]	177,707	[1]	179,430	[1]	183,033	[1]	184,864	[1]	722,013	725,034	779,495
Equity in earnings or loss of unconsolidated entities, gains or losses on dispositions and other, net																	(829)	2,723	17,478
Continuing operations																	(20,047)	(1,959)	(858)
Noncontrolling interests in consolidated real estate partnerships																	24,595	12,473	51,218
Net income (loss) attributable to the company's common equityholders	36,269	[1]	124,706	[1]	75,010	[1]	64,235	[1]	122,037	[1]	66,268	[1]	10,107	[1]	5,050	[1]	300,220	203,673	82,146
Napico Portfolio [Member] | Other Nonoperating Income (Expense) [Member]
Significant Acquisitions and Disposals [Line Items]
Revenues																	27,701	23,711
Expenses																	21,472	21,188
Equity in earnings or loss of unconsolidated entities, gains or losses on dispositions and other, net																	(6,996)	(748)
Net (loss) income related to legacy asset management business																	(767)	1,775
Continuing operations																	(3)	639
Noncontrolling interests in consolidated real estate partnerships																	403	(21,370)
Net income (loss) attributable to the company's common equityholders																	$ (1,167)	$ 22,506

[1] Certain reclassifications have been made to quarterly amounts to conform to the full year presentation.