Financial Instruments Financial Instruments - Natural Gas Swap Agreements (Detail) - Natural Gas Commodity [Member] MMBTU in Millions, $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) MMBTU $ / MMBTU	Dec. 31, 2014 USD ($) MMBTU	Dec. 31, 2013 USD ($)
Derivative [Line Items]
Percentage of hedged twelve month exposure to fluctuations in natural gas prices	28.00%	6.00%	27.00%
Net (payments) receipts under derivative swap agreements \| $	$ (1.2)	$ 0.1	$ (0.2)
Derivative, Nonmonetary Notional Amount \| MMBTU	1.4	1.0
Natural Gas Commodity [Member] \| Minimum [Member]
Derivative [Line Items]
Underlying, Derivative	2.91
Natural Gas Commodity [Member] \| Maximum [Member]
Derivative [Line Items]
Underlying, Derivative	4.17

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

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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.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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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

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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

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