Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment information
Number of reportable segments | segment									2
Total revenues	$ 238,283	$ 237,651	$ 234,631	$ 214,668	$ 214,378	$ 212,164	$ 204,515	$ 216,465	$ 925,233	$ 847,522	$ 821,822
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate, gain on sale of real estate and depreciable real estate reserves									272,572	198,104	190,896
Total assets	8,858,747				8,303,773				$ 8,858,747	8,303,773
Leverage rate assumption (as a percent)									100.00%
Equity in net gain on sale of interest in unconsolidated joint venture					0	0	86	85,473	$ 0	85,559	2,056
Gain on sale of real estate	(879)	101,069	0	0					100,190	0	0
Depreciable real estate reserves	0	(9,998)	0	0					(9,998)	0	0
Income from continuing operations									362,764	283,663	192,952
Net income from discontinued operations									0	1,996	5,907
Gain on sale of discontinued operations					0	(250)	117,829	0	0	117,579	13,756
Net income	72,338	$ 157,108	$ 78,389	$ 54,929	47,251	$ 50,173	$ 166,336	$ 139,478	362,764	403,238	212,615
Operating Segments | Real Estate Segment
Segment information
Total revenues									742,585	670,621	627,210
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate, gain on sale of real estate and depreciable real estate reserves									112,564	52,431	25,038
Total assets	6,816,711				6,671,984				6,816,711	6,671,984
Operating Segments | Debt and Preferred Equity Segment
Segment information
Total revenues									182,648	176,901	194,612
Income from continuing operations before equity in net gain on sale of interest in unconsolidated joint venture/real estate, gain on sale of real estate and depreciable real estate reserves									160,008	145,673	$ 165,858
Total assets	$ 2,042,036				$ 1,631,789				$ 2,042,036	$ 1,631,789