Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014
Asset-backed loans, by major industry:
Total asset-backed loans	$ 82.0		$ 82.0			$ 274.6
Less: Allowance for credit losses	33.8		33.8			47.9
Total asset-backed loans, net	48.2		48.2			226.7
Financing Receivable, Allowance for Credit Losses	33.8	$ 32.1	33.8	$ 32.1	$ 47.8	47.9	$ 22.7	$ 5.5
Provision for Loan, Lease, and Other Losses	11.7	$ (9.4)	(2.7)	$ (77.7)
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	34.9		34.9			66.0
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	16.8		16.8			36.9
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	11.9		11.9			32.7
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	13.2		13.2			93.1
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 5.2		$ 5.2			$ 45.9