Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Jun. 30, 2013	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Jun. 30, 2013
Asset-backed loans, by major industry:
Total asset-backed loans	$ 433.3	$ 181.5
Less: Allowance for credit losses	3.1	1.4
Total asset-backed loans, net	430.2	180.1
Wholesale [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	23.0	77.2
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	177.5	70.1
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	59.8	27.9
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 173.0	$ 6.3