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Asset Retirement Obligations (Details) (USD $)
In Thousands
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
ARO Operating Costs    
Accretion$ 1,894$ 2,081$ 6,189$ 6,525
Depreciation1,9473,0505,3429,390
Total3,8415,13111,53115,915
Asset Retirement Obligations    
Balance at beginning of period160,733162,168162,730167,757
Liabilities incurred1,4561,0161,7342,457
Liabilities settled(6,238)(4,762)(12,202)(8,879)
Accretion expense1,8942,0816,1896,525
Revisions up (down)139(288)(467)(7,645)
Balance at end of period$ 157,984$ 160,215$ 157,984$ 160,215