Investments - Schedule of Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Document Period End Date			Mar. 31, 2014
Allowance for credit losses:
Balance at beginning of period	$ 7.0	$ 2.6	$ 5.2	$ 1.4
Provision for credit losses		0.2	1.8	1.4
Balance at end of period	$ 7.0	$ 2.8	$ 7.0	$ 2.8