Earnings per Share and Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Document Fiscal Year Focus			2015
Document Period End Date			Sep. 30, 2015
Earnings Per Share [Abstract]
Gain on sale of real estate	$ 27,755	$ 27,558	$ 34,215	$ 29,598
Income per common unit - diluted
Weighted Average Limited Partnership Units Outstanding, Basic	154,170,000	158,920	154,170	159,229
Parent Company [Member]
Earnings Per Share [Abstract]
Income from operations	$ 31,979	$ 25,882	$ 94,789	$ 80,061
Gain on sale of real estate	27,755	27,558	34,215	29,598
Less: income attributable to noncontrolling interests	737	232	1,823	1,048
Income from operations attributable to the Company	58,997	53,208	127,181	108,611
Preferred stock dividends	$ (5,266)	$ (5,415)	$ (15,797)	$ (16,245)
Weighted average common units outstanding for basic EPU (in shares)	94,158,000	92,345,000	94,080,000	92,071,000
Weighted average common units outstanding for diluted EPU (in shares)	94,595,000	92,396,000	94,483,000	92,107,000
Income per common share/unit - basic:
Continuing operations (in dollars per share)	$ 0.57	$ 0.52	$ 1.18	$ 1.00
Income per common share/unit - diluted:
Continuing operations (in dollars per share)	$ 0.57	$ 0.52	$ 1.18	$ 1.00
Partnership Interest [Member]
Earnings Per Share [Abstract]
Income from operations	$ 31,979	$ 25,882	$ 94,789	$ 80,061
Gain on sale of real estate	27,755	27,558	34,215	29,598
Less: income attributable to noncontrolling interests	643	142	1,619	863
Income from operations attributable to the Company	59,091	53,298	127,385	108,796
Less: preferred unit distributions and other	$ 5,266	$ 5,415	$ 15,797	$ 16,245
Weighted average common units outstanding for basic EPU (in shares)	94,312,000	92,505,000	94,234,000	92,231,000
Weighted average common units outstanding for diluted EPU (in shares)	94,749,000	92,556,000	94,637,000	92,267,000
Income per common unit - basic
Continuing operations (in dollars per share)	$ 0.57	$ 0.52	$ 1.18	$ 1.00
Income per common unit - diluted
Continuing operations (in dollars per share)	$ 0.57	$ 0.52	$ 1.18	$ 1.00
Continuing Operations [Member] | Parent Company [Member]
Earnings Per Share [Abstract]
Net income attributable to common stockholders	$ 53,731	$ 47,793	$ 111,384	$ 92,366
Net income for common stock unit/holders - diluted	53,731	47,817	111,384	92,416
Continuing Operations [Member] | Partnership Interest [Member]
Earnings Per Share [Abstract]
Net income attributable to common stockholders	53,825	47,883	111,588	92,551
Net income for common stock unit/holders - diluted	$ 53,825	$ 47,907	$ 111,588	$ 92,601