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Investments - Available-for-Sale Securities (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Sep. 30, 2015
Schedule of Investments [Line Items]                
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value $ 49.1 $ 64.5 $ 49.1 $ 64.5 $ 46.4 $ 78.1 $ 65.1 $ 91.6
Total Gains (Losses) Included in Earnings   (0.3) (1.3) 0.3        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 6.1 3.1 9.1 8.1        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 1.0 $ 3.4 7.3 30.0        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements       5.5        
Net transfer In (Out) of Level 3 (a) (2.4) [1]   (29.5) [2]          
Corporate Debt Securities [Member]                
Schedule of Investments [Line Items]                
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value               $ 14.1
Total Gains (Losses) Included in Earnings       (0.5)        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales       $ 13.6        
Fixed-maturity securities, available-for-sale [Member]                
Schedule of Investments [Line Items]                
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 20.3   20.3     46.2    
Available-for-sale Securities, Debt Securities 1,543.4   1,543.4     1,535.1    
Total investments, Carrying Value 1,543.4   1,543.4     1,535.1    
Available-for-sale Debt Securities, Amortized Cost Basis $ 1,566.8   $ 1,566.8     $ 1,531.8    
[1] (a) During the three months ended March 31, 2017, the net transfer out of Level 3 was exclusively to Level 2.
[2] (a) During the six months ended March 31, 2017, the net transfer out of Level 3 was exclusively to Level 2.