Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Asset-backed loans, by major industry:
Total asset-based loans	$ 818.8	$ 565.6	$ 565.6
Less: Allowance for loan losses	7.2	5.2
Total asset-based loans, net	811.6	560.4
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-based loans	245.4
Transportation [Member]
Asset-backed loans, by major industry:
Total asset-based loans	44.3	85.7
Wholesale
Asset-backed loans, by major industry:
Total asset-based loans	191.6	252.9
Apparel
Asset-backed loans, by major industry:
Total asset-based loans	100.1	125.8
Jewelry
Asset-backed loans, by major industry:
Total asset-based loans	13.9	25.1
Other
Asset-backed loans, by major industry:
Total asset-based loans	94.9	41.8
Home Furnishings [Member]
Asset-backed loans, by major industry:
Total asset-based loans	71.7
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-based loans	$ 56.9	$ 34.3