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Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits	$ 16.7		$ 16.7		$ 30.4
Unrecognized tax benefits that would impact effective tax rate	16.7		16.7
Uncertain tax positions related to alternative fuel mixture credits	9.9
Lower range of gross unrecognized tax benefits balance may decrease within the next twelve months	0		0
Higher range of gross unrecognized tax benefits balance may decrease within the next twelve months	5.9		5.9
Interest expense	0.4	0.3	0.1	0.7
Liability for interest	1.1		1.1		1.4
Penalties associated with uncertain tax positions			0	0
Return to Internal Revenue Service		$ 16.8		$ 16.8

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Additional Income Tax Payment

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The amount of estimated interest recognized in the period arising from income tax examinations.

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The estimated lower bound of the reasonably possible change in the total amount of the unrecognized tax benefit (that will significantly increase or decrease within twelve months of the balance sheet date) for a tax position taken.

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The estimated upper bound of the reasonably possible change in the total amount of the unrecognized tax benefit (that will significantly increase or decrease within twelve months of the balance sheet date) for a tax position taken.

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Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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Amount of expense for penalties related to a tax position claimed or expected to be claimed in the tax return.

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Amount of interest expense accrued for an underpayment of income taxes.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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