Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date			Mar. 31, 2014
Asset-backed loans, by major industry:
Total asset-backed loans	$ 802.7		$ 802.7			$ 565.6
Less: Allowance for credit losses	7.0		7.0			5.2
Total asset-backed loans, net	795.7		795.7			560.4
Financing Receivable, Allowance for Credit Losses	7.0	2.8	7.0	2.8	7.0	5.2	2.6	1.4
Provision for Loan, Lease, and Other Losses		0.2	1.8	1.4
Wholesale [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	225.5		225.5			252.9
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	121.0		121.0			125.8
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	16.9		16.9			25.1
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	53.6		53.6			34.3
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	90.1		90.1			41.8
Transportation [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	45.6		45.6			85.7
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 250.0		$ 250.0