Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Sep. 30, 2015
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	$ 82.0	$ 274.6
Substandard [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	11.2	74.0
Doubtful [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	69.5	99.2
Special Mention [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	$ 1.3	32.4
Pass [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross		$ 69.0