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Gross Realized Gains and Losses on Sales and OTTI on Securities Available for Sale (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Available-for-sale Securities, Gross Realized Gains $ 0.0 $ 10.5 $ 4.0 $ 24.5  
Available-for-sale Securities 29,672.0   29,672.0   $ 27,825.0
Available-for-sale Securities, Gross Realized Losses   (3.1)   (3.1)  
Gain (Loss) on Sale of Securities, Net 0.0 (7.0) (4.0) (21.0)  
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment $ 0.0 $ 0.0 $ 0.0 $ 0.0  
Document Period End Date     Sep. 30, 2016