Schedule III - Real Estate and Accumulated Depreciation (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Beginning Balance, Total Property					$ 2,831,732
Retirement/dispositions:
Ending Balance, Total Property	3,271,536		3,084,166	2,830,931	2,831,732
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Beginning Balance, Accumulated Depreciation					536,682
Retirement/dispositions:
Ending Balance, Accumulated Depreciation	705,135		642,320	586,920	536,682
Real Estate [Member]
Additions during the period:
Total Property	166,290		314,159	104,810
Retirement/dispositions:
Total Property	(74,507)		(111,656)	(128,325)
Ending Balance, Total Property	3,249,006	[1],[2]
Additions during the period:
Accumulated Depreciation	2,272		1,046	1,079
Retirement/dispositions:
Accumulated Depreciation	(46,276)		(42,927)	(45,117)
Ending Balance, Accumulated Depreciation	701,301	[2],[3]
Other improvements [Member]
Additions during the period:
Total Property	55,340		58,849	55,299
Additions during the period:
Accumulated Depreciation	105,257		97,255	94,250
Corporate Property [Member]
Additions during the period:
Total Property	40,247		0	0
Retirement/dispositions:
Ending Balance, Total Property	5,476	[1],[2]
Additions during the period:
Accumulated Depreciation	1,536		0	0
Retirement/dispositions:
Ending Balance, Accumulated Depreciation	3,719	[2],[3]
Land held for development [Member]
Additions during the period:
Total Property	0		0	0
Retirement/dispositions:
Total Property	0		(8,117)	0
Additions during the period:
Accumulated Depreciation	26		26	26
Retirement/dispositions:
Accumulated Depreciation	0		0	0
Construction in Progress [Member]
Additions during the period:
Total Property	0		0	5,608
Additions during the period:
Accumulated Depreciation	0		0	0
Disposal of previously consolidated VIE [Member]
Retirement/dispositions:
Total Property	0		0	(38,193)
Retirement/dispositions:
Accumulated Depreciation	$ 0		$ 0	$ 0

[1] Total assets as of December 31, 2014 have an estimated aggregate total cost of $3.3 billion for federal income tax purposes.

[2] Depreciation is provided for on a straight-line basis on buildings and improvements over 3.3 to 39.0 years, lease intangibles over 1.0 to 93.1 years, personal property over 1.9 to 15.8 years, and land improvements over 15.0 to 38.1 years.

[3] Includes two assets held for sale as of December 31, 2014 of approximately $13.3 million (gross) and accumulated depreciation of $4.5 million, three asset held for sale as of December 31, 2013 of approximately $17.0 million (gross) and accumulated depreciation of $10.2 million; and one assets held for sale as of December 31, 2012 of $9.6 million (gross) and accumulated depreciation of $6.3 million.