Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 segment	Dec. 31, 2013	Dec. 31, 2012
Segment information
Number of reportable segments (segment)									2
Total revenues	$ 386,627,000	$ 390,274,000	$ 380,631,000	$ 362,446,000	$ 350,451,000	$ 338,782,000	$ 340,888,000	$ 340,944,000	$ 1,519,978,000	$ 1,371,065,000	$ 1,290,052,000
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate and purchase price fair value adjustment									173,030,000	109,400,000	133,780,000
Total assets	17,096,587,000				14,959,001,000				17,096,587,000	14,959,001,000
Leverage rate assumption (as a percent)									100.00%
Marketing, general and administrative expenses and transaction related costs									92,500,000	86,200,000	82,800,000
Real Estate Segment
Segment information
Total revenues									1,341,163,000	1,177,222,000	1,170,897,000
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate and purchase price fair value adjustment									22,178,000	(49,793,000)	41,366,000
Total assets	15,671,662,000				13,641,727,000				15,671,662,000	13,641,727,000
Debt and Preferred Equity Segment
Segment information
Total revenues									178,815,000	193,843,000	119,155,000
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate and purchase price fair value adjustment									150,852,000	159,193,000	92,414,000
Total assets	$ 1,424,925,000				$ 1,317,274,000				$ 1,424,925,000	$ 1,317,274,000