Financial Instruments Financial Instruments - Natural Gas Swap Agreements (Detail) (Commodity Contract [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014 MMBTU	Dec. 31, 2013 MMBTU	Dec. 31, 2012
Derivative [Line Items]
Percentage of hedged twelve month exposure to fluctuations in natural gas prices	6.00%	27.00%	21.00%
Net receipts (payments) under derivative swap agreements	$ 0.1	$ (0.2)	$ (1.2)
Aggregate notional principal amount (MMBTU) of natural gas swap agreements	1,000,000	100,000
Natural Gas Commodity [Member] \| Minimum [Member]
Derivative [Line Items]
Underlying, Derivative	4.01
Natural Gas Commodity [Member] \| Maximum [Member]
Derivative [Line Items]
Underlying, Derivative	4.55

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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- Details

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