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Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2017
Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Net transfer In (Out) of Level 3 (a) $ (2.4) [1] $ (29.5) [2]  
Fixed Maturities Available For Sale [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Available-for-sale Securities, Debt Securities $ 1,543.4 $ 1,543.4 $ 1,535.1
[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.