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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Property, Plant and Equipment Impairment or Disposal [Abstract]
Revenues	$ 436,000	$ 5,281,000	$ 3,955,000	$ 19,663,000
Operating expenses	(656,000)	(2,173,000)	(1,634,000)	(8,049,000)
Interest and other income	3,000	304,000	13,000	643,000
Interest expense	(135,000)	(583,000)	(491,000)	(3,481,000)
Depreciation and amortization	(320,000)	(1,000,000)	(1,103,000)	(2,072,000)
Income (loss) before gain on property dispositions	(672,000)	1,829,000	740,000	6,704,000
Gain on property dispositions	7,625,000	2,981,000	49,338,000	4,045,000
Income from discontinued operations	6,953,000	4,810,000	50,078,000	10,749,000
Proceeds from Disposition of Operating Properties	$ 51,400,000	$ 204,300,000	$ 126,000,000	$ 210,800,000