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Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Realized gain/(loss) on sale of securities $ 20.2 $ 8.8 $ 43.8 $ 16.6
Tax (expense) benefit (1.7) (2.7) 0.4 (2.4)
Total reclassifications for the period 12.7 5.0 27.3 8.8
Unrealized Gains and Losses on Available-for-Sale Securities [Member]        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period 13.2 5.7 28.5 10.8
Benefit Plan Items [Member]        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period (0.5) (0.7) (1.2) (2.0)
Reclassification Out of Accumulated Other Comprehensive Income [Member]        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period 12.7 5.0 27.3 8.8
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Realized gain/(loss) on sale of securities 20.2 8.8 43.8 16.6
Total before tax 20.2 8.8 43.8 16.6
Tax (expense) benefit 7.0 3.1 15.3 5.8
Net of tax 13.2 5.7 28.5 10.8
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Benefit Plan Items [Member]        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Accumulated other comprehensive income components/Negative prior-service costs 1.3 1.3 4.1 4.1
Accumulated other comprehensive income components/Net actuarial loss (2.1) (2.4) (6.1) (7.1)
Total before tax (0.8) (1.1) (2.0) (3.0)
Tax (expense) benefit (0.3) (0.4) (0.8) (1.0)
Net of tax $ (0.5) $ (0.7) $ (1.2) $ (2.0)