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

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

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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