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Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Detail) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]    
Document Period End Date Dec. 31, 2016  
Fixed Maturities Available For Sale [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total fixed maturity available-for-sale securities, Amortized Cost $ 1,570.1 $ 1,531.8
Total investments, Carrying Value 1,532.8 1,535.1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax 18.9 46.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax 56.2 42.9
Available-for-sale Securities, Debt Securities $ 1,532.8 $ 1,535.1