2011 New Tax rate

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2011 Original UK Tax rate

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2011 Previous UK Tax rate

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2012 UK tax rate

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Income Taxes [Line Items]

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The non-cash component of income tax expense due to change in tax law.

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Ultimate UK Tax rate

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Unrecognized Tax Benefits, Reasonably Possible Decreases Resulting From Settlements With Taxing Authorities

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Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

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Amount of the required periodic payments applied to principal.

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The aggregate tax effects as of the balance sheet date of all future tax deductions arising from temporary differences between tax basis and generally accepted accounting principles basis recognition of assets, liabilities, revenues and expenses, which can only be deducted for tax purposes when permitted under enacted tax laws; net of deducting the allocated valuation allowance, if any, to reduce such amount to net realizable value.

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Gross amount of debt extinguished.

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The portion of the difference between total income tax expense or benefit as reported in the Income Statement and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations, that is attributable to changes in the income tax rates under enacted tax laws in the period.

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The gross amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns as of the balance sheet date.

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This element represents the total of interest expense recognized for an underpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity and the amount of statutory penalties in the period in which the entity claims or expects to claim a tax position, in its tax return, that does not meet the minimum statutory threshold to avoid payment of penalties.

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Other - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Sep. 30, 2010	Mar. 31, 2011	Mar. 31, 2011 7.625% senior notes due March 2011	Sep. 30, 2011 Northwind Holdings LLC	Sep. 30, 2011 Tailwind Holdings LLC	Sep. 30, 2011 Minimum	Sep. 30, 2011 Maximum
Income Taxes [Line Items]
Principal payments on senior secured non-recourse variable rate notes								$ 59.4	$ 7.5
Debt maturing in 2011							225.1
Cash Collateral for Borrowed Securities	342.4			342.4
Liability for unrecognized tax benefits	133.5			133.5
Liability for unrecognized tax benefits, associated with deferred tax assets	118.3			118.3
Total unrecognized tax benefit that would impact, if recognized, the effective tax rate	15.2			15.2
Interest expense and penalties related to unrecognized tax expense	1.5	1.5		4.1	4.0
Decrease in liability for unrecognized tax benefits										0	55.0
2011 Original UK Tax rate						28.00%
Ultimate UK Tax rate	23.00%			23.00%
2011 Previous UK Tax rate						27.00%
2011 New UK Tax rate	26.00%			26.00%
2012 UK tax rate	25.00%			25.00%
Income Tax Reconciliation, Change in Enacted Tax Rate				6.8	2.7
Income tax expense, non-cash tax charge			$ 10.2