Investments - Schedule of Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Investments, Debt and Equity Securities [Abstract]
Document Period End Date			Jun. 30, 2014
Allowance for credit losses:
Balance at beginning of period	$ 7.0	$ 2.8	$ 5.2	$ 1.4
Provision for credit losses	(0.3)	0.3	1.5	1.7
Balance at end of period	$ 6.7	$ 3.1	$ 6.7	$ 3.1