Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Jun. 30, 2013	Sep. 30, 2012
Financing Receivable, Recorded Investment [Line Items]
Document Period End Date	Jun. 30, 2013
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 433.3	$ 181.5
Total asset-backed loans, percent	100.00%	100.00%
Not Rated [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans
Total asset-backed loans, percent
Credit Rating AAA/AA/A [Member] | Asset-based loans, by credit quality Rating 1 [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	19.8	75.8
Total asset-backed loans, percent	4.60%	41.70%
Credit Rating BBB [Member] | Asset-based loans, by credit quality Rating 2 [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	392.8	94.9
Total asset-backed loans, percent	90.70%	52.30%
Credit Rating B [Member] | Asset-based loans, by credit quality Rating 3 [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans		10.8
Total asset-backed loans, percent		6.00%
Credit Rating B [Member] | Asset-based loans, by credit quality Rating 4 [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans
Total asset-backed loans, percent
Credit Rating CCC [Member] | Asset-based loans, by credit quality Rating 5 [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 20.7
Total asset-backed loans, percent	4.70%