Investments - Schedule of Allowance for Credit Losses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Total asset-based loans	$ 818.8	$ 565.6	$ 565.6
Allowance for Credit Losses on Asset Backed Securities [Roll Forward]
Balance at beginning of year	5.2	1.4		5.2
Provision for credit losses	2.0	3.8	1.4
Balance at end of year	7.2	5.2	1.4	5.2
Pass [Member]
Schedule of Available-for-sale Securities [Line Items]
Total asset-based loans	195.3	306.9
Special Mention [Member]
Schedule of Available-for-sale Securities [Line Items]
Total asset-based loans	372.7	36.7
Substandard [Member]
Schedule of Available-for-sale Securities [Line Items]
Total asset-based loans	250.8	222.0
Doubtful [Member]
Schedule of Available-for-sale Securities [Line Items]
Total asset-based loans