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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 |  |
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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 |  |  |  |  |  |  |  |

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