Quarterly Financial Data (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Selected Quarterly Financial Information [Line Items]
Revenues	$ 131,918,000	[1]	$ 129,434,000	[1]	$ 123,163,000	[1]	$ 119,023,000	[1]	$ 119,729,000	[1]	$ 122,851,000	[1]	$ 120,964,000	[1]	$ 114,813,000	[1]	$ 503,538,000	$ 478,357,000	$ 471,284,000
Net income	54,975,000	[2],[3]	42,047,000	[2],[3]	32,761,000	[2],[3],[4]	22,638,000	[4]	32,570,000		(35,958,000)	[4]	2,939,000	[4]	17,188,000		152,421,000	16,739,000	51,238,000
Net income (loss) attributable to common shareholders	42,928,000	[2],[3]	31,404,000	[2],[3]	22,550,000	[2],[3],[4]	12,328,000	[4]	22,173,000		(42,089,000)	[4]	(7,166,000)	[4]	7,227,000		109,210,000	(19,855,000)	10,730,000
Earnings per common share - basic	$ 0.36	[2],[3]	$ 0.26	[2],[3]	$ 0.19	[2],[3],[4]	$ 0.10	[4]	$ 0.18		$ (0.35)	[4]	$ (0.06)	[4]	$ 0.06		$ 0.90	$ (0.17)	$ 0.09
Earnings per common share - diluted	$ 0.35	[2],[3]	$ 0.26	[2],[3]	$ 0.19	[2],[3],[4]	$ 0.10	[4]	$ 0.18		$ (0.35)	[4]	$ (0.06)	[4]	$ 0.06		$ 0.90	$ (0.17)	$ 0.09
Net impact of impairment charges					24,900,000		10,000,000				52,600,000		21,300,000				35,382,000	78,437,000	33,432,000
Gains on sale of properties and real estate joint ventures and partnership interests and on acquisitions	$ 27,700,000		$ 17,000,000		$ 31,300,000

[1] Revenues from the sale of operating properties have been reclassified and reported in discontinued operations for all periods presented.

[2] During the second quarter of 2012, we disposed of our wholly-owned Industrial portfolio.

[3] The quarter results include significant gains on the sale of properties and real estate joint venture and partnership interests and on acquisitions. Gain amounts are: $31.3 million, $17.0 million and $27.7 million for the three months ended June 30, 2012, September 30, 2012 and December 31, 2012, respectively.

[4] The quarter results include significant impairment charges. Impairment amounts are: $10.0 million and $24.9 million for the three months ended March 31, 2012 and June 30, 2012, respectively, and $21.3 million and $52.6 million for the three months ended June 30, 2011 and September 30, 2011, respectively.