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, 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	Dec. 31, 2011	Dec. 31, 2010
Income from Continuing Operations:
Income from continuing operations-basic	$ 29,014	$ 21,492	$ 6,299	$ 17,653	$ 3,862	$ 1,356	$ 11,654	$ 25,632
Net Income Available for Common Shares-Fully Diluted:
Net income available for Common Shares-basic									54,778	22,775	38,354
Amounts allocated to dilutive securities									5,067	3,571	5,903
Net income available for Common Shares-fully diluted									$ 59,845	$ 26,346	$ 44,257
Denominator:
Weighted average Common Shares outstanding-basic	41,285	41,190	41,131	41,088	40,263	38,346	32,629	30,996	41,174	35,591	30,517
Effect of dilutive securities:
Redemption of Common OP Units for Common Shares									3,938	4,260	4,730
Redemption of Series B Preferred Stock									0	153	0
Employee stock options and restricted shares									319	326	271
Weighted average Common Shares outstanding-fully diluted	45,472	45,447	45,390	45,369	45,296	43,602	37,262	35,609	45,431	40,330	35,518
Net income available for Common Shares	$ 0.59	$ 0.39	$ 0.05	$ 0.30	$ 0.00	$ (0.07)	$ 0.21	$ 0.61	$ 1.33	$ 0.64	$ 1.26
Net income available for Common Shares	$ 0.58	$ 0.39	$ 0.05	$ 0.30	$ 0.00	$ (0.07)	$ 0.20	$ 0.61	$ 1.32	$ 0.64	$ 1.25