Quarterly Financial Data (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Selected Quarterly Financial Information [Line Items]
Revenues	$ 128,183,000	[1]	$ 125,451,000	[1]	$ 124,217,000	[1]	$ 119,874,000	[1]	$ 119,547,000	[1]	$ 117,807,000	[1]	$ 111,712,000	[1]	$ 107,838,000	[1]	$ 497,725,000	$ 456,904,000	$ 432,616,000
Net income	53,772,000	[2]	62,389,000	[2]	104,178,000	[2]	44,817,000	[2],[3]	54,975,000	[2]	42,047,000	[2]	32,761,000	[2],[4],[5]	22,638,000	[4],[5]	265,156,000	152,421,000	16,739,000
Net income attributable to common shareholders	47,224,000	[2]	57,832,000	[2]	45,421,000	[2],[6]	33,668,000	[2],[3]	42,928,000	[2]	31,404,000	[2]	22,550,000	[2],[4],[5]	12,328,000	[4],[5]	184,145,000	109,210,000	(19,855,000)
Earnings per common share - basic	$ 0.39	[2]	$ 0.48	[2]	$ 0.37	[2],[6]	$ 0.28	[2],[3]	$ 0.36	[2]	$ 0.26	[2]	$ 0.19	[2],[4],[5]	$ 0.10	[4],[5]	$ 1.52	$ 0.90	$ (0.17)
Earnings per common share - diluted	$ 0.38	[2]	$ 0.47	[2]	$ 0.37	[2],[6]	$ 0.28	[2],[3]	$ 0.35	[2]	$ 0.26	[2]	$ 0.19	[2],[4],[5]	$ 0.10	[4],[5]	$ 1.50	$ 0.90	$ (0.17)
Gain on sale of properties and real estate joint ventures and partnership interests and on acquisitions	25,200,000		38,400,000		78,400,000		11,700,000		27,700,000		17,000,000		31,300,000
Net income attributable to noncontrolling interests					37,700,000												44,894,000	5,781,000	1,118,000
Redemption costs of preferred shares																	17,944,000	2,500,000	0
Net impact of impairment charges													24,900,000		10,000,000		3,210,000	35,382,000	78,437,000
6.5% Series F Preferred Shares [Member]
Schedule of Selected Quarterly Financial Information [Line Items]
Redemption costs of preferred shares					15,700,000												15,700,000
Acceleration Of Mortgage Intangible Amortization [Member]
Schedule of Selected Quarterly Financial Information [Line Items]
Amortization (accretion)							$ (9,700,000)										$ (9,700,000)

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

[2] 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: $11.7 million, $78.4 million, $38.4 million and $25.2 million for the three months ended March 31, 2013, June 30, 2013, September 30, 2013 and December 31, 2013, respectively, and $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.

[3] The quarter results include a write-off of an above-market assumed mortgage intangible due to the early payoff of the related mortgage of $9.7 million.

[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.

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

[6] The quarter results include net income attributable to noncontrolling interests of $37.7 million associated with applicable gains discussed in (2) above and a $15.7 million deduction associated with the redemption of the Series F preferred shares (see Note 9 for additional information).