Investments - Schedule of Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Mar. 31, 2015	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013
Allowance for credit losses:
Balance at beginning of period	$ 7.5	$ 7.2	$ 5.2	$ 7.0
Provision for credit losses	102.9	103.2	1.8
Balance at end of period	$ 33.8	$ 33.8	$ 7.0	$ 7.0