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Financial Instruments Financial Instruments - Natural Gas Swap Agreements (Detail) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 MMBTU | Dec. 31, 2012 MMBTU | Dec. 31, 2011
Derivative [Line Items] |  |  |
Percentage of hedged estimated twelve month exposure to fluctuations in natural gas prices | 27.00% | 21.00% | 29.00%
Net payments under derivative swap agreements | $ 0.2 | $ 1.2 | $ 0.6
Aggregate notional principal amount of natural gas swap agreements | 100,000 | 1,000,000 |
Natural Gas Commodity [Member] | Minimum [Member] |  |  |
Derivative [Line Items] |  |  |
Underlying, Derivative | 3.45 |  |
Natural Gas Commodity [Member] | Maximum [Member] |  |  |
Derivative [Line Items] |  |  |
Underlying, Derivative | 4.470 |  |

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition The premium (cost) of a hedge, net of any cash received on settlement during the period. + References No definition available. + Details
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- Details
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- Definition The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need. + References No definition available. + Details
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- Definition Specified interest rate, security price, commodity price, foreign exchange rate, index of prices or rates for a derivative contract but not the derivative contract itself. + References No definition available. + Details
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