Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2014	Sep. 30, 2013
Financing Receivable, Recorded Investment [Line Items]
Document Period End Date	Mar. 31, 2014
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 802.7	$ 565.6
Doubtful [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	0	0
Substandard [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	245.7	222.0
Special Mention [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	335.6	36.7
Pass [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 221.4	$ 306.9