Investments - Reconciliation of Other than Temporary Impairment on Fixed Maturity (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2010
Investments, Debt and Equity Securities [Abstract]
Beginning balance	$ 2.7	$ 2.7	$ 2.7	$ 2.7	$ 0.7
Increases attributable to credit losses on securities:
Other-than-temporary impairment was previously recognized	0	0	0.1
Other-than-temporary impairment was not previously recognized	0	0	1.9
Ending balance	$ 2.7	$ 2.7	$ 2.7	$ 2.7	$ 0.7