Annual report pursuant to Section 13 and 15(d)

Contingencies - Additional Information (Detail)

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Contingencies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2012
Environmental Issue
Plea Agreement Fine
Dec. 31, 2012
Environmental Issue
Community Service Payment
Sep. 30, 2010
Royalties
Dec. 31, 2009
Royalties
Dec. 31, 2012
Cameron Parish Louisiana Claim
Dec. 31, 2012
Contingent Liability Recorded
Dec. 31, 2011
Contingent Liability Recorded
Dec. 31, 2010
Contingent Liability Recorded
Long-term Purchase Commitment [Line Items]                  
Plea agreement fine payment, which is subject to federal district court approval   $ 0.7 $ 0.3            
Claims paid           10.0      
Accrued claims           1.3      
Expected claims under excess liability policies 50.0                
Allowable reductions of cash payments         5.3        
Notified disallowed amount in reductions taken by ONRR       4.7          
Expenses related to accrued and settled claims             9.3 1.7 0.7
Liability loss contingency             $ 1.3 $ 2.0