Schedule III - Real Estate and Accumulated Depreciation (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Real Estate and Accumulated Depreciation [Line Items]
Land	$ 691,726,000	$ 638,071,000
Buildings and Improvements	2,403,654,000	2,326,245,000
Less: Accumulated Depreciation	732,635,000	658,729,000
Subtotal	2,362,745,000
Construction in Progress	26,068,000	27,780,000
Net Investment in Real Estate	2,388,813,000	2,333,367,000
Total at December 31, 2012	6,765,000	91,659,000
Tenant Improvements	Life of lease
Aggregate cost of land and buildings and equipment for federal income tax purpose	3,100,000,000
Balance, Beginning of Year	3,115,050,000	3,140,649,000	3,351,626,000
Acquisition of Real Estate Assets	65,770,000	22,953,000	17,595,000
Construction Costs and Improvements	74,116,000	72,822,000	49,881,000
Disposition of Real Estate Assets	(94,093,000)	(91,312,000)	(50,929,000)
Impairment of Real Estate	(1,246,000)	2,661,000	(194,552,000)
Write-off of Fully Depreciated Assets	(28,655,000)	(32,723,000)	(32,972,000)
Balance, End of Year	3,130,942,000	3,115,050,000	3,140,649,000
Balance, Beginning of Year	695,931,000	663,310,000	597,461,000
Depreciation for Year	100,074,000	95,931,000	104,175,000
Disposition of Assets	(31,757,000)	(30,587,000)	(5,354,000)
Balance, End of Year	735,593,000	695,931,000	663,310,000
Gross Amount Carried At Close of Period [Member]
Real Estate and Accumulated Depreciation [Line Items]
Land	694,116,000
Buildings and Improvements	2,410,758,000
Less: Accumulated Depreciation	(735,593,000)
Subtotal	2,369,281,000
Construction in Progress	26,068,000
Net Investment in Real Estate	2,395,349,000
Assets Held-for-sale [Member]
Real Estate and Accumulated Depreciation [Line Items]
Land	2,390,000
Buildings and Improvements	7,104,000
Less: Accumulated Depreciation	2,958,000
Subtotal	6,536,000
Net Investment in Real Estate	6,536,000
Deferred Rent Receivable, Net and Other Assets, Net	$ 229,000
Buildings and Improvements [Member] | Minimum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Property, plant and equipment, useful life	7 years
Buildings and Improvements [Member] | Maximum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Property, plant and equipment, useful life	50 years
Land Improvements [Member] | Minimum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Property, plant and equipment, useful life	3 years
Land Improvements [Member] | Maximum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Property, plant and equipment, useful life	20 years