Summary of Discontinued Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental revenues	$ 113,783	$ 97,996	$ 222,111	$ 192,517
Other revenues	9,930	8,942	19,506	17,761
Total operating revenues	123,922	107,201	242,095	210,764
Property operating expenses	82,457	73,002	161,901	143,523
Interest expense	14,270	13,945	28,486	27,704
(Loss) income from discontinued operations before gain on sale	63	641	154	1,255
Discontinued Operation, Asset Impairment	0	0	(71)	0
Net loss on insurance and other settlement proceeds on discontinued operations	(2)	0	(56)	(7)
Gain on sale of discontinued operations	12,953	0	22,453	0
Income from discontinued operations	13,014	641	22,480	1,248
Discontinued Operation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rental revenues	1,069	3,376	2,651	6,740
Other revenues	123	411	304	825
Total operating revenues	1,192	3,787	2,955	7,565
Property operating expenses	623	2,070	1,637	4,119
Depreciation and amortization	373	873	879	1,754
Interest expense	133	203	285	437
Total expense	$ 1,129	$ 3,146	$ 2,801	$ 6,310