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Schedule of Changes in Level Three Securities (Detail) (Fair Value, Significant Unobservable Inputs (Level 3), USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Fair Value, Significant Unobservable Inputs (Level 3)
 
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Balance at beginning of period $ 29.8
Total realized and unrealized gains or (losses) Included in net income   
Total realized and unrealized gains or (losses) included in OCI (4.4)
Accretion of impairments included in net income 0.4
Sales   
Purchases, issuances and settlements   
Balance at end of period $ 25.8