Earnings per Share and Unit (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income per common unit - diluted
Weighted Average Limited Partnership Units Outstanding, Basic	160,281	177,164
Parent Company [Member]
Earnings Per Share [Abstract]
Income from continuing operations	$ 24,341	$ 18,511
Gain on sale of real estate	715	0
Less: income (loss) attributable to noncontrolling interests	401	309
Income from continuing operations attributable to the Company	24,655	18,202
Preferred stock dividends	(5,266)	(5,266)
Less: dividends paid on unvested restricted stock	197	216
Net income attributable to common stockholders	19,192	15,338
Add: dividends paid on Treasury Method restricted stock	(11)	(28)
Net income for common stock unit/holders - diluted	19,203	15,366
Income from discontinued operations	0	2,623
Less: income from discontinued operations attributable to noncontrolling interests	0	5
Income from discontinued operations attributable to the Company	0	2,618
Weighted average common units outstanding for basic EPU (in shares)	92,167,000	90,290,000
Incremental units to be issued under unvested restricted stock (in shares)	24,000	61,000
Weighted average common units outstanding for diluted EPU (in shares)	92,191,000	90,351,000
Income per common share/unit - basic:
Continuing operations (in dollars per share)	$ 0.21	$ 0.14
Discontinued operations (in dollars per share)	$ 0.00	$ 0.03
Net income attributable to common stockholders (in dollars per share)	$ 0.21	$ 0.17
Income per common share/unit - diluted:
Continuing operations (in dollars per share)	$ 0.21	$ 0.14
Discontinued operations (in dollars per share)	$ 0.00	$ 0.03
Net income attributable to common stockholders (in dollars per share)	$ 0.21	$ 0.17
Partnership Interest [Member]
Earnings Per Share [Abstract]
Income from continuing operations	24,341	18,511
Gain on sale of real estate	715	0
Income from continuing operations attributable to the Company	24,697	18,241
Less: dividends paid on unvested restricted stock	197	216
Net income attributable to common stockholders	19,234	15,377
Add: dividends paid on Treasury Method restricted stock	(11)	(28)
Net income for common stock unit/holders - diluted	19,245	15,405
Income from discontinued operations	0	2,623
Less: income from discontinued operations attributable to noncontrolling interests	0	5
Income from discontinued operations attributable to the Company	0	2,618
Less: preferred unit distributions	5,266	5,266
Weighted average common units outstanding for basic EPU (in shares)	92,168,000	90,289,000
Incremental units to be issued under unvested restricted stock (in shares)	24,000	60,000
Weighted average common units outstanding for diluted EPU (in shares)	92,192,000	90,349,000
Income per common unit - basic
Continuing operations (in dollars per share)	$ 0.21	$ 0.14
Discontinued operations (in dollars per share)	$ 0.00	$ 0.03
Net income attributable to common unit holders (in dollars per share)	$ 0.21	$ 0.17
Income per common unit - diluted
Continuing operations (in dollars per share)	$ 0.21	$ 0.14
Discontinued operations (in dollars per share)	$ 0.00	$ 0.03
Net income attributable to common unit holders (in dollars per share)	$ 0.21	$ 0.17
Continuing Operations [Member] | Parent Company [Member]
Earnings Per Share [Abstract]
Net income attributable to common stockholders	19,192	12,720
Net income for common stock unit/holders - diluted	19,203	12,748
Continuing Operations [Member] | Partnership Interest [Member]
Earnings Per Share [Abstract]
Less: income (loss) attributable to noncontrolling interests	359	270
Net income attributable to common stockholders	19,234	12,759
Net income for common stock unit/holders - diluted	$ 19,245	$ 12,787