Accounting for the Impairment or Disposal of Long-Lived Assets - Impairment (Details) (USD $)	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Jun. 30, 2011 Central Region [Member]	Jun. 30, 2011 Central Region [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charges	$ 537,000	$ 594,000	$ 4,200,000	$ 4,700,000