Accounting for Derivative Instruments and Hedging Activities - Derivatives Designated as Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current liabilities	$ 14.6	$ 58.4
Long-term derivative liabilities	0.6	8.6
Total liabilities	15.2	67.0
Current assets	0	0.9
Long-term derivative assets	0.2	0
Total assets	0.2	0.9
Diesel Fuel Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	0.5	0
Long-term derivative liabilities	0.4	1.2
Total liabilities	0.9	1.2
Current assets	0	0.9
Long-term derivative assets	0	0
Total assets	0	0.9
Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	14.1	58.4
Long-term derivative liabilities	0.2	7.4
Total liabilities	14.3	65.8
Current assets	0	0
Long-term derivative assets	0.2	0
Total assets	0.2	0
Derivatives Designated As Hedging Instruments [Member]
Derivatives, Fair Value [Line Items]
Total liabilities	15.2	67.0
Total assets	0.2	0.9
Derivatives Designated As Hedging Instruments [Member] | Diesel Fuel Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	0.5	0
Long-term derivative liabilities	0.4	1.2
Current assets	0	0.9
Long-term derivative assets	0	0
Derivatives Designated As Hedging Instruments [Member] | Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	14.1	58.4
Long-term derivative liabilities	0.2	7.4
Current assets	0	0
Long-term derivative assets	$ 0.2	$ 0