Segment Information (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 segment	Dec. 31, 2011	Dec. 31, 2010
Segment Information
Number of reportable segments									2
Segment information
Total revenues:	$ 350,733,000	$ 361,385,000	$ 349,004,000	$ 339,133,000	$ 328,877,000	$ 306,624,000	$ 298,705,000	$ 329,222,000	$ 1,400,255,000	$ 1,263,428,000	$ 1,084,386,000
Income from continuing operations before equity in net gain on sale of unconsolidated joint venture/ partial interest and purchase price fair value adjustments									156,904,000	124,144,000	147,686,000
Total assets:	14,387,754,000				13,483,852,000				14,387,754,000	13,483,852,000
Leverage rate assumption (as a percent)									100.00%
Marketing, general and administrative expenses and transaction related costs									88,500,000	85,700,000	87,800,000
Real Estate Segment
Segment information
Total revenues:									1,281,100,000	1,143,010,000	936,460,000
Income from continuing operations before equity in net gain on sale of unconsolidated joint venture/ partial interest and purchase price fair value adjustments									63,958,000	23,107,000	27,101,000
Total assets:	13,021,095,000				12,490,502,000				13,021,095,000	12,490,502,000
Debt and Preferred Equity Segment
Segment information
Total revenues:									119,155,000	120,418,000	147,926,000
Income from continuing operations before equity in net gain on sale of unconsolidated joint venture/ partial interest and purchase price fair value adjustments									92,946,000	101,037,000	120,585,000
Total assets:	$ 1,366,659,000				$ 993,350,000				$ 1,366,659,000	$ 993,350,000