Activity in Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Year	$ 1.5	$ 1.5	$ 3.2
Provisions	1.8	0	2.4
Charge-offs, Net of Recoveries	(1.8)	0	(4.1)
Balance at End of Period	$ 1.5	$ 1.5	$ 1.5