Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012
Asset-backed loans, by major industry:
Total asset-backed loans	$ 244.4	$ 181.5
Less: Allowance for credit losses	2.8	1.4
Total asset-backed loans, net	241.6	180.1
Wholesale [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	66.0	77.2
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	96.7	70.1
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	39.2	27.9
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 42.5	$ 6.3