Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	$ 196.8	$ 274.6
Substandard [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	43.8	74.0
Doubtful [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	101.7	99.2
Special Mention [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	39.6	32.4
Pass [Member]
Asset-based loans, by credit quality rating
Asset Backed Loans And Other Invested Assets Gross	$ 11.7	$ 69.0