Quarterly report pursuant to Section 13 or 15(d)

Contingencies - Additional Information (Detail)

v2.4.0.6
Contingencies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 3 Months Ended 3 Months Ended
Mar. 31, 2013
Dec. 31, 2009
Royalties
Sep. 30, 2010
Royalties
Mar. 31, 2013
Contingent Liability Recorded
Mar. 31, 2012
Contingent Liability Recorded
Dec. 31, 2012
Contingent Liability Recorded
Mar. 31, 2013
Cameron Parish Louisiana Claim
Dec. 31, 2012
Cameron Parish Louisiana Claim
Mar. 31, 2013
Hurricane Ike
Mar. 31, 2013
Hurricane Ike
Future Costs
Apr. 30, 2013
Subsequent Event
Cameron Parish Louisiana Claim
Loss Contingencies [Line Items]                      
Claims paid               $ 10.0     $ 0.5
Accrued claims             1.3 1.3      
Expected claims under excess liability policies 50.0                    
Costs incurred related to damages by hurricane                 49.7 0.7  
Allowable reductions of cash payments   5.3                  
Notified disallowed amount in reductions taken by ONRR     4.7                
Expenses related to accrued and settled claims       0 8.3            
Liability loss contingency       $ 1.3   $ 1.3