Annual report pursuant to Section 13 and 15(d)

Capitalized Costs Related to Oil and Natural Gas (Detail)

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Capitalized Costs Related to Oil and Natural Gas (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Net capitalized cost:      
Proved oil and natural gas properties and equipment $ 6,551.5 $ 5,775.4 $ 5,130.9
Unproved oil and natural gas properties and equipment 143.0 183.6 94.7
Accumulated depreciation, depletion and amortization related to oil, NGLs and natural gas activities (4,640.8) (4,307.1) (4,009.9)
Net capitalized costs related to producing activities $ 2,053.7 $ 1,651.9 $ 1,215.7