Activity in Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at Beginning of Year	$ 2.0	$ 1.5	$ 1.5	$ 1.5
Provisions	0	0	0.5	0
Charge-offs, Net of Recoveries	(0.5)	0	(0.5)	0
Balance at End of Period	$ 1.5	$ 1.5	$ 1.5	$ 1.5