Accounting for Derivative Instruments and Hedging Activities - Derivatives Designated as Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Derivative assets	$ 0.2	$ 9.7
Long-term derivative assets	0	0.3
Derivative assets, Net Amounts of Assets	0.2	10.0
Derivative liabilities	4.1	0.1
Long-term liabilities	1.6	0.2
Total liabilities	5.7	0.3
Derivative assets	0	0.3
Derivative liabilities	1.6	0.2
Natural Gas Contracts [Member]
Derivatives Fair Value [Line Items]
Derivative assets	0.1	9.5
Long-term derivative assets	0	0.3
Derivative assets, Net Amounts of Assets	0.1	9.8
Derivative liabilities	4.0	0
Long-term liabilities	1.6	0.2
Total liabilities	5.6	0.2
Derivative assets	0	0.3
Derivative liabilities	1.6	0.2
Derivatives Designated as Hedging Instruments [Member]
Derivatives Fair Value [Line Items]
Derivative assets, Net Amounts of Assets	0.2	10.0
Total liabilities	5.7	0.3
Diesel Fuel Derivatives [Member]
Derivatives Fair Value [Line Items]
Derivative assets	0.1	0.2
Long-term derivative assets	0	0
Derivative assets, Net Amounts of Assets	0.1	0.2
Derivative liabilities	0.1	0.1
Long-term liabilities	0	0
Total liabilities	0.1	0.1
Derivative assets	0	0
Derivative liabilities	0	0
Diesel Fuel Derivatives [Member] | Derivatives Designated as Hedging Instruments [Member]
Derivatives Fair Value [Line Items]
Derivative assets	0.1	0.2
Long-term derivative assets	0	0
Derivative liabilities	0.1	0.1
Long-term liabilities	0	0
Derivative assets	0	0
Derivative liabilities	0	0
Natural Gas Derivatives [Member] | Derivatives Designated as Hedging Instruments [Member]
Derivatives Fair Value [Line Items]
Derivative assets	0.1	9.5
Long-term derivative assets	0	0.3
Derivative liabilities	4.0	0
Long-term liabilities	1.6	0.2
Derivative assets	0	0.3
Derivative liabilities	$ 1.6	$ 0.2