Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Sep. 30, 2015
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	$ 184.6	$ 274.6
Substandard [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	17.1	74.0
Doubtful [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	122.4	99.2
Special Mention [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	33.0	32.4
Pass [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	$ 12.1	$ 69.0