Earnings per Share and Unit Earnings per Share and Unit (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income (Loss) Attributable to Common Unit Holders		$ 8,165	$ 8,537	$ 12,898	$ 2,198	$ (37,971)	$ 7,928	$ 7,775	$ 11,493
Weighted Average Limited Partnership Units Outstanding, Basic										177,164	270,706	468,211
Earnings Per Share [Abstract]
Net income attributable to common stockholders	842	8,139	8,510	12,861	2,185	(37,900)	7,894	7,748	11,399
Net income attributable to common stockholders		$ 0.10	$ 0.09	$ 0.14	$ 0.02	$ (0.47)	$ 0.10	$ 0.09	$ 0.14
Net income attributable to common stockholders		$ 0.10	$ 0.09	$ 0.14	$ 0.02	$ (0.46)	$ 0.09	$ 0.09	$ 0.14
Partnership Interest [Member]
Earnings Per Share [Abstract]
Income from continuing operations										48,649	1,192	(38,813)
Discontinued operations										7,139	11,809	9,777
Net income										55,788	13,001	(29,036)
Less: Noncontrolling interests										590	376	452
Less: Dividends paid on unvested restricted stock										615	542	488
Net income attributable to common stock/unit holders - basic										31,183	(11,317)	(53,376)
Add: dividends paid on treasury method restricted stock										18	0	0
Net income for common stock unit/holders - diluted										31,201	(11,317)	(53,376)
Weighted average common shares/units outstanding for basic EPS/EPU										88,002	81,339	76,908
Incremental shares under Forward Equity Offering										424	1,534	0
Weighted average common shares/units outstanding for diluted EPS/EPU										88,426	82,873	76,908
Earnings Per Unit [Abstract]
Income from continuing operations										48,649	1,192	(38,813)
Discontinued operations										7,139	11,809	9,777
Net income										55,788	13,001	(29,036)
Preferred unit distributions										23,400	23,400	23,400
Less: Noncontrolling interests										590	376	452
Net income attributable to common unit holders										31,798	(10,775)	(52,888)
Less: Dividends paid on unvested restricted stock										615	542	488
Add: dividends paid on treasury method restricted stock										18	0	0
Incremental units under Forward Equity Offering										424	1,534	0
Continuing operations										0.27	(0.29)	(0.82)
Discontinued operations										0.08	0.15	0.12
Net income attributable to common unit holders										0.35	(0.14)	(0.70)
Continuing operations										0.27	(0.28)	(0.82)
Discontinued operations										0.08	0.14	0.12
Net income attributable to common unit holders										0.35	(0.14)	(0.70)
Parent Company [Member]
Earnings Per Share [Abstract]
Income from continuing operations										48,649	1,192	(38,813)
Discontinued operations										7,139	11,809	9,777
Net income										55,788	13,001	(29,036)
Less: Preferred stock dividends										19,675	19,675	19,675
Less: Noncontrolling interests										4,418	4,185	3,961
Net income attributable to common stockholders										31,695	(10,859)	(52,672)
Less: Dividends paid on unvested restricted stock										615	542	488
Net income attributable to common stock/unit holders - basic										31,080	(11,401)	(53,160)
Add: dividends paid on treasury method restricted stock										18	0	0
Net income for common stock unit/holders - diluted										31,098	(11,401)	(53,160)
Weighted average common shares/units outstanding for basic EPS/EPU										87,825	81,068	76,440
Incremental shares under Forward Equity Offering										424	1,534	67
Weighted average common shares/units outstanding for diluted EPS/EPU										88,249	82,602	76,507
Continuing operations										0.27	(0.29)	(0.82)
Discontinued operations										0.08	0.15	0.12
Net income attributable to common stockholders										0.35	(0.14)	(0.70)
Continuing operations										0.27	(0.28)	(0.82)
Discontinued operations										0.08	0.14	0.12
Net income attributable to common stockholders										0.35	(0.14)	(0.70)
Earnings Per Unit [Abstract]
Income from continuing operations										48,649	1,192	(38,813)
Discontinued operations										7,139	11,809	9,777
Net income										55,788	13,001	(29,036)
Less: Noncontrolling interests										4,418	4,185	3,961
Less: Dividends paid on unvested restricted stock										615	542	488
Add: dividends paid on treasury method restricted stock										18	0	0
Incremental units under Forward Equity Offering										424	1,534	67