Investments - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Long-term Debt		$ 6,353.8		$ 6,353.8		$ 6,439.1
Asset-backed loans		$ 136.8		$ 136.8		226.7
Industry Holdings, Number of Top Investments		10		10
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments		10.00%		10.00%
Total investments, Carrying Value		$ 139.3		$ 139.3		$ 278.9
Financing Receivable, Allowance for Credit Losses, Write-downs			$ 51.1		$ 51.1
Financing Receivable, Allowance for Credit Losses, Period Increase (Decrease)		(4.1)	(17.4)	0.1	(17.2)
Provision for Loan, Lease, and Other Losses		6.7	$ 68.5	15.1	$ 68.3
Radioshack [Member]
Schedule of Available-for-sale Securities [Line Items]
Financing Receivable, Allowance for Credit Losses, Write-downs		$ 2.6		$ 15.2
Fair Value, Concentration of Risk, All Financial Instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage Of Investments Fair Value In Relation To Invested Assets Portfolio		32.60%		32.60%		16.50%
Estimate of Fair Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset-backed loans	[1]	$ 136.8		$ 136.8		$ 226.7
Loan Participations and Assignments [Member] | Radioshack [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term Debt						50.0
Radioshack [Member] | Salus Capital Partners L L C [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term Debt		250.0		250.0		250.0
Radioshack [Member] | Salus Capital Partners L L C [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term Debt						93.0
Radioshack [Member] | Front Street RE (Cayman) Ltd. [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term Debt						7.0
Non-Qualifying Participation [Member] | Radioshack [Member]
Schedule of Available-for-sale Securities [Line Items]
Long-term Debt						100.0
Doubtful [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset Backed Loans And Other Invested Assets Gross		$ 104.0		$ 104.0		$ 79.8

[1] The carrying amounts of cash and cash equivalents, trade receivables, accounts payable and accrued investment income approximate fair value due to their short duration and, accordingly, they are not presented in the tables above.