Accounting for the Impairment or Disposal of Long-Lived Assets - Results of Operations for the Properties Held for Sale and Disposed (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Long Lived Assets Held-for-sale [Line Items]
Revenues	$ 1,634,000	$ 12,404,000
Operating expenses	(179,000)	(4,686,000)
Interest and other income	0	29,000
Interest expense	(148,000)	(2,711,000)
Depreciation and amortization	(427,000)	(713,000)
Income before gain on property dispositions	880,000	4,323,000
Gain on property dispositions	41,713,000	1,064,000
Income from discontinued operations	42,593,000	5,387,000
Proceeds from Disposition of Operating Properties	74,700,000	6,500,000
Asset held for sale, Sold subsequent to reporting period [Member]
Long Lived Assets Held-for-sale [Line Items]
Number Properties Held For Sale	1
Property Square Feet Held For Sale	86,000
Long Lived Assets Held-for-sale, Proceeds from Sale	$ 11,000,000