Accounting For Derivative Instruments And Hedging Activities (Derivatives Designated As Hedging Instruments) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current liabilities	$ 40.5	[1]	$ 58.4	[1]
Long-term derivative liabilities	3.9	[1]	8.6	[1]
Total liabilities	44.4	[1]	67.0	[1]
Current assets	0.7	[1]	0.9	[1]
Long-term derivative assets	0.1	[1]	0	[1]
Total assets	0.8	[1]	0.9	[1]
Diesel Fuel Contracts [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	1.6		0
Long-term derivative liabilities	1.3		1.6
Total liabilities	2.9		1.2
Current assets	0.3		0.9
Long-term derivative assets	0		0
Total assets	0.3		0.9
Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	38.9		58.4
Long-term derivative liabilities	2.6		7.4
Total liabilities	41.5		65.8
Current assets	0.4		0
Long-term derivative assets	0.1		0
Total assets	0.5		0
Designated As Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total liabilities	44.4
Total assets	0.8
Designated As Hedging Instrument [Member] | Diesel Fuel Contracts [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	1.6
Long-term derivative liabilities	1.3
Current assets	0.3
Long-term derivative assets	0
Designated As Hedging Instrument [Member] | Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current liabilities	38.9
Long-term derivative liabilities	2.6
Current assets	0.4
Long-term derivative assets	$ 0.1

[1] Amounts presented above are on a gross basis, with asset and liability positions netted by counterparty in accordance with accounting standards for derivatives and hedging.