Accounting for the Impairment or Disposal of Long-Lived Assets - Results of Operations for the Properties Held for Sale and Disposed (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues	$ 12,203	$ 62,181	$ 98,090
Operating expenses	(5,551)	(27,838)	(40,787)
Interest and other income	36	325	370
Interest expense	(3,516)	(10,328)	(16,753)
Depreciation and amortization	(634)	(14,273)	(24,542)
Income before property dispositions	2,538	10,067	16,378
Gain on property dispositions	11,383	60,582	6,857
Net Income	$ 13,921	$ 70,649	$ 23,235