Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Sep. 30, 2013
Financing Receivable, Recorded Investment [Line Items]
Document Period End Date	Dec. 31, 2013
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 801.0	$ 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	257.5	222.0
Special Mention [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	338.8	36.7
Pass [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 204.7	$ 306.9