Summary of Discontinued Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental revenues	$ 116,968	$ 101,004	$ 335,091	$ 289,630
Other revenues	9,933	9,052	28,965	26,354
Total operating revenues	127,110	110,321	364,743	316,735
Property operating expenses	84,816	76,034	243,258	216,130
Interest expense	14,698	15,183	42,978	42,681
(Loss) income from discontinued operations before gain on sale	(451)	805	500	2,777
Net loss on insurance and other settlement proceeds on discontinued operations	99	0	43	(7)
Gain on sale of discontinued operations	16,092	4,927	38,474	4,927
Income from discontinued operations	15,740	5,732	39,017	7,697
Discontinued Operation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental revenues	1,155	5,584	7,791	15,745
Other revenues	128	675	907	1,909
Total operating revenues	1,283	6,259	8,698	17,654
Property operating expenses	1,176	3,632	5,150	9,697
Depreciation and amortization	416	1,516	2,414	4,227
Interest expense	142	306	634	953
Total expense	$ 1,734	$ 5,454	$ 8,198	$ 14,877