Schedule III - Real Estate and Accumulated Depreciation (Details 1) (USD $) In Thousands, unless otherwise specified					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Real Estate [Member]		Dec. 31, 2011 Real Estate [Member]	Dec. 31, 2010 Real Estate [Member]	Dec. 31, 2012 Corporate Property [Member]		Dec. 31, 2011 Corporate Property [Member]	Dec. 31, 2010 Corporate Property [Member]	Dec. 31, 2012 Land held for development [Member]	Dec. 31, 2011 Land held for development [Member]	Dec. 31, 2010 Land held for development [Member]	Dec. 31, 2012 Construction in Progress [Member]	Dec. 31, 2011 Construction in Progress [Member]	Dec. 31, 2010 Construction in Progress [Member]	Dec. 31, 2012 Disposal of previously consolidated VIE [Member]
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Beginning Balance, Total Property	$ 2,841,297	$ 2,841,453	$ 2,614,557	$ 2,250,879
Additions during the period:
Total Property					159,963		187,158	337,223	794		885	316	0	4,403	3,471	5,608	101,282	59,830
Retirement/dispositions:
Total Property					(128,109)		(65,682)	(37,155)	(219)		(1,150)	(7)							(38,193)
Ending Balance, Total Property	2,841,297	2,841,453	2,614,557	2,250,879	2,801,089	[1],[2],[3]			15,037	[1],[2],[3]
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Beginning Balance, Accumulated Depreciation	593,028	541,304	504,088	442,331
Additions during the period:
Accumulated Depreciation					95,260		85,440	72,825	1,557		1,429	740	26	0	0	0	0	0
Retirement/dispositions:
Accumulated Depreciation					(44,898)		(48,507)	(11,801)	(221)		(1,146)	(7)
Ending Balance, Accumulated Depreciation	$ 593,028	$ 541,304	$ 504,088	$ 442,331

[1] Total assets as of December 31, 2012 have an estimated aggregate total cost of $2.8 billion for federal income tax purposes.

[2] A reconciliation of Total Property and Accumulated Depreciation for the twelve months ended December 31, 2012, 2011 and 2010 follows:

[3] Depreciation is provided for on a straight-line basis on buildings and improvements over 3.3 to 39.0 years, lease intangibles over 2.0 to 93.1 years, personal property over 3.0 to 15.8 years, and land improvements over 15.0 to 38.1 years.