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Activity in Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Year	$ 1.5	$ 1.5
Provisions	0	0.5
Balance at End of Period	$ 1.5	$ 2.0

The sum of the periodic provision charged to earnings, based on an assessment of uncollectibility from the counterparty on account of loan, lease or other credit losses, to reduce these accounts to the amount that approximates their net realizable value.

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