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Rollforward of Credit Losses Recognized in Earnings Related to Securities (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]      
Document Period End Date Dec. 31, 2016    
Available-for-sale Securities, Gross Realized Gains $ 4.0 $ 24.5 $ 28.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment 0.0 1.0 1.0
Available-for-sale Securities 30,672.0 27,825.0  
Beginning balance 25.0 25.0  
Ending balance 23.0 25.0 25.0
Available-for-sale Securities, Gross Realized Losses $ 0.0 $ (3.0) $ (42.0)