Derivative Financial Instruments - Summary of Gain (Loss) Recognized in Income on Derivatives (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Commodity contracts [Member] | Other (expense) income, net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Three months ended December 31,	$ 1.7	$ 18.7
Foreign Exchange Contract [Member] | Other (expense) income, net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Three months ended December 31,	(2.1)	(1.7)
Call options [Member] | Net investment gains [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Three months ended December 31,	1.9	2.7
Embedded Derivatives in Reinsurance Contracts [Member] | Net investment gains [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Three months ended December 31,	$ 2.4	$ (6.2)