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Related Party Transactions - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Sep. 30, 2016
Related Party Transaction [Line Items]          
Long-term Debt $ 6,133.6   $ 6,133.6   $ 5,635.6
Fixed Maturities [Member] | Fortress Investment Group [Member]          
Related Party Transaction [Line Items]          
Related Party Transaction, Due from (to) Related Party, Noncurrent 175.9   175.9   $ 203.2
Net Investment Income $ 3.0 $ 2.5 $ 9.1 $ 7.0