Quarterly Results of Operations (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
Summary of Unaudited Quarterly Results of Operations
Revenues as reported and adjusted																	$ 2,880,608,000	$ 1,805,044,000	$ 1,313,182,000
Net income (loss) attributable to common stockholders	11,473,000		20,691,000		(8,508,000)	[1]	55,058,000		90,576,000		37,269,000	[2]	54,735,000		39,307,000		78,714,000	221,884,000	157,108,000
Net income (loss) attributable to common stockholders per share:
Basic	$ 0.04		$ 0.07		$ (0.03)	[1]	$ 0.21		$ 0.35		$ 0.17	[2]	$ 0.26		$ 0.2		$ 0.28	$ 0.99	$ 0.9
Diluted	$ 0.04		$ 0.07		$ (0.03)	[1]	$ 0.21		$ 0.35		$ 0.16	[2]	$ 0.25		$ 0.19		$ 0.28	$ 0.98	$ 0.9
Quarterly Results of Operations (Textuals) [Abstract]
Decreases in net income amounts and per share attributable to gains on sales of real estate					(29,997,000)		82,492,000				12,827,000		32,450,000
Impairment of assets																	0	29,287,000	12,194,000
Scenario, Previously Reported [Member]
Summary of Unaudited Quarterly Results of Operations
Revenues as reported and adjusted	788,577,000		786,930,000		682,125,000	[1]	633,915,000		500,663,000		474,139,000	[2]	453,082,000		435,359,000
Scenario, Adjustment [Member]
Summary of Unaudited Quarterly Results of Operations
Discontinued operations	0		(3,217,000)		(3,592,000)	[1]	(4,129,000)		(3,699,000)		(15,120,000)	[2]	(17,821,000)		(21,559,000)
Scenario, Actual [Member]
Summary of Unaudited Quarterly Results of Operations
Revenues as reported and adjusted	$ 788,577,000	[3]	$ 783,713,000	[3]	$ 678,533,000	[1],[3]	$ 629,786,000	[3]	$ 496,964,000	[3]	$ 459,019,000	[2],[3]	$ 435,261,000	[3]	$ 413,800,000	[3]

[1] The decrease in net income and amounts per share are primarily attributable to gains on sales of real estate of $82,492,000 for the first quarter as compared to losses of $29,997,000 for the second quarter.

[2] The decrease in net income and amounts per share are primarily attributable to gains on sales of real estate of $32,450,000 for the second quarter as compared to $12,827,000 for the third quarter.

[3] We have reclassified the income attributable to the properties sold prior to or held for sale at December 31, 2013 to discontinued operations. See Note 5.