Investments - Additional Information (Detail) $ in Millions		6 Months Ended
		Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Document Fiscal Year Focus		2017
Long-term Debt		$ 5,727.0	$ 5,540.7
Industry Holdings, Number of Top Investments		10
Fixed Maturities Available For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Total investments, Carrying Value		$ 1,543.4	1,535.1
Estimate of Fair Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset-backed loans	[1]	$ 6.3	$ 35.0

[1] The carrying value 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.