Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Dec. 31, 2013
Asset-backed loans, by major industry:
Total asset-backed loans	$ 801.0		$ 565.6
Less: Allowance for credit losses	7.0		5.2
Total asset-backed loans, net	794.0		560.4
Financing Receivable, Allowance for Credit Losses	7.0	2.6	5.2	1.4
Provision for Loan, Lease, and Other Losses	1.8	1.2
Wholesale [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	235.7		252.9
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	117.1		125.8
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	16.3		25.1
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	55.8		34.3
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	83.6		41.8
Transportation [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	42.5		85.7
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 250.0