Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Asset-backed loans, by major industry:
Total asset-backed loans	$ 196.8		$ 274.6
Less: Allowance for credit losses	43.7		47.9
Total asset-backed loans, net	153.1		226.7
Financing Receivable, Allowance for Credit Losses	43.7	$ 5.3	47.9	$ 5.5
Provision for Loan, Lease, and Other Losses	7.9	$ (0.2)
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	55.5		66.0
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	36.3		36.9
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	30.4		32.7
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	29.1		93.1
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 45.5		$ 45.9