Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014	Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Dec. 31, 2014
Asset-backed loans, by major industry:
Total asset-backed loans	$ 799.4		$ 818.8
Less: Allowance for credit losses	7.5		7.2
Total asset-backed loans, net	791.9		811.6
Financing Receivable, Allowance for Credit Losses	7.5	7.0	7.2	5.2
Provision for Loan, Lease, and Other Losses	0.3	1.8
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	198.2		191.6
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	89.6		100.1
Home Furnishings [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	57.2		71.7
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	54.1		56.9
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	98.3		94.9
Transportation [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	45.3		44.3
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	245.7		245.4
451110 Sporting Goods Stores [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 11.0		$ 13.9