Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date			Jun. 30, 2014
Asset-backed loans, by major industry:
Total asset-backed loans	$ 731.0		$ 731.0			$ 565.6
Less: Allowance for credit losses	6.7		6.7			5.2
Total asset-backed loans, net	724.3		724.3			560.4
Financing Receivable, Allowance for Credit Losses	6.7	3.1	6.7	3.1	7.0	5.2	2.8	1.4
Provision for Loan, Lease, and Other Losses	(0.3)	0.3	1.5	1.7
Wholesale [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	169.2		169.2			252.9
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	99.0		99.0			125.8
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	13.0		13.0			25.1
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	60.2		60.2			34.3
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	93.7		93.7			41.8
Transportation [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	45.9		45.9			85.7
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 250.0		$ 250.0