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Commitments, Contingencies and Legal Proceedings - Additional Information (Detail) (USD $)
 9 Months Ended 0 Months Ended 0 Months Ended 1 Months Ended
Sep. 30, 2014
Sep. 30, 2014
Maximum [Member]
Sep. 30, 2014
Minimum [Member]
Oct. 14, 2014
Subsequent Event [Member]
Oct. 14, 2014
Subsequent Event [Member]
Oct. 14, 2014
OU1 [Member]
Subsequent Event [Member]
Sep. 25, 2014
OU2-5 [Member]
Sep. 30, 2014
OU2-5 [Member]
Jan. 31, 2008
OU2-5 [Member]
Whiting Litigation [Member]
Oct. 14, 2014
OU2-5 [Member]
Subsequent Event [Member]
Sep. 30, 2014
Dredging Season [Member]
Commitments Contingencies And Litigation [Line Items]                      
Upper end estimate of total past and future response costs       $ 1,100,000,000              
Uncertainty premium for future response costs       100,000,000              
Estimated future cost of work yet to be done           100,000,000       500,000,000  
Annual estimated cost rate seek by Government               70,000,000     90,000,000
Initial minimum amount of NRD assessment 176,000,000                    
Initial maximum amount of NRD assessment 333,000,000                    
Revised minimum amount of NRD assessment 287,000,000                    
Revised maximum amount of NRD assessment 423,000,000                    
Reserve for environmental liability, expected compensation payments       45,900,000              
Environmental loss contingencies recovered 16,300,000       105,000,000            
Other prepaid loss contingencies for earlier settlements         59,000,000            
Cost of response incurred by government 4,280,000                    
NCR costs and damages percentage             100.00%   100.00%    
Reserve for Environmental liabilities, current portion 100,000                    
Maximum estimated percentage of discharge 27.00%                    
Maximum estimated cost related to Fox River matter $ 185,000,000                    
Period over which estimated cost accrued   P3Y P2Y