Computation of Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	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
Income from Continuing Operations:
Income from continuing operations-basic	$ 3,863	$ 1,355	$ 11,654	$ 25,632	$ 10,596	[1]	$ 17,307	[1]	$ 11,021	[1]	$ 21,704	[1]	$ 42,504	$ 60,628		$ 51,395
Amounts allocated to dilutive securities													3,571	5,935		5,433
Income from continuing operations-fully diluted													46,075	66,563		56,828
(Loss) income from Discontinued Operations:
(Loss) income from discontinued operations-basic														(199)		4,186
Amounts allocated to dilutive securities														(32)		680
(Loss) income from discontinued operations									(54)	[1]	(177)	[1]		(231)	[2]	4,866	[3]
Net Income Available for Common Shares-Fully Diluted:
Net income available for Common Shares-basic	(159)	(2,853)	6,827	18,960	5,736		11,554		6,000		15,064		22,775	38,354		34,005
Amounts allocated to dilutive securities													3,571	5,903		6,113
Net income available for Common Shares-fully diluted													$ 26,346	$ 44,257		$ 40,118
Denominator:
Weighted average Common Shares outstanding-basic	40,263	38,346	32,629	30,996	30,728		30,620		30,412		30,304		35,591	30,517		27,582
Effect of dilutive securities:
Redemption of Common OP Units for Common Shares													4,260	4,730		5,075
Redemption of Series B Preferred Stock													153
Employee stock options and restricted shares													326	271		287
Weighted average Common Shares outstanding-fully diluted	45,296	43,602	37,262	35,609	35,597		35,530		35,506		35,465		40,330	35,518		32,944
Earnings per Common Share-Basic:
Income from continuing operations available for Common Shares													$ 0.64	$ 1.26		$ 1.08
Income from discontinued operations																$ 0.15
Net income available for Common Shares	$ 0.00	$ (0.07)	$ 0.21	$ 0.61	$ 0.19		$ 0.38		$ 0.20		$ 0.50		$ 0.64	$ 1.26		$ 1.23
Earnings per Common Share-Fully Diluted:
Income from continuing operations available for Common Shares													$ 0.64	$ 1.25		$ 1.07
Income from discontinued operations																$ 0.15
Net income available for Common Shares	$ 0.00	$ (0.07)	$ 0.20	$ 0.61	$ 0.18		$ 0.37		$ 0.20		$ 0.49		$ 0.64	$ 1.25		$ 1.22

[1] Amounts may differ from previously disclosed amounts due to reclassification of discontinued operations.

[2] For the year ended December 31, 2010, includes one Property disposed of in January 2010.

[3] For the year ended December 31, 2009, includes one Property sold in July 2009 and one Property disposed of in January 2010.