Activity in Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Year		$ 1.5	$ 3.2	$ 0
Provisions	0	0	2.4	5.5
Charge-offs, Net of Recoveries	0	0	(4.1)	(2.3)
Balance at End of Period	$ 1.5	$ 1.5	$ 1.5	$ 3.2