Investments - Schedule of Credit Quality of Asset-Based Loan Portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012
Asset-based loans, by credit quality rating
Total asset-backed loans	$ 244.4	$ 181.5
Total asset-backed loans, percent	100.00%	100.00%
Asset-based loans, by credit quality Rating 1 [Member] | Credit Rating AAA/AA/A [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	18.0	75.8
Total asset-backed loans, percent	7.40%	41.70%
Asset-based loans, by credit quality Rating 2 [Member] | Credit Rating BBB [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	200.2	94.9
Total asset-backed loans, percent	81.90%	52.30%
Asset-based loans, by credit quality Rating 3 [Member] | Credit Rating B [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans		10.8
Total asset-backed loans, percent		6.00%
Asset-based loans, by credit quality Rating 4 [Member] | Credit Rating B [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans
Total asset-backed loans, percent
Asset-based loans, by credit quality Rating 5 [Member] | Credit Rating CCC [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans	26.2
Total asset-backed loans, percent	10.70%
Not Rated [Member]
Asset-based loans, by credit quality rating
Total asset-backed loans
Total asset-backed loans, percent