Investments - Portfolio of Asset-Backed Loans and Other Invested Assets (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Asset-backed loans, by major industry:
Total asset-backed loans	$ 184.6		$ 184.6			$ 274.6
Less: Allowance for credit losses	47.8		47.8			47.9
Total asset-backed loans, net	136.8		136.8			226.7
Financing Receivable, Allowance for Credit Losses	47.8	$ 22.7	47.8	$ 22.7	$ 43.7	47.9	$ 5.3	$ 5.5
Provision for Loan, Lease, and Other Losses	(6.7)	$ (68.5)	(15.1)	$ (68.3)
Apparel [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	50.3		50.3			66.0
Jewelry [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	35.6		35.6			36.9
Manufacturing [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	29.9		29.9			32.7
Other [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	23.4		23.4			93.1
Electronics [Member]
Asset-backed loans, by major industry:
Total asset-backed loans	$ 45.4		$ 45.4			$ 45.9