Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Sep. 30, 2014
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Other	$ 0.7	$ 2.9
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Including Credit Impairments	0.6	0.8	5.7
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Change Of Intent Operations	0.1	2.2	17.1
Amortized Cost Of Investments With Other Than Temporary Impairments	1.8	9.6		162.3
Fair value on other than temporary impairments	1.1	6.7		138.0
Change-of-intent, and non-credit losses in other comprehensive income		1.5
Total other-than-temporary impairments	0.7		22.8
Non-agency residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains included in AOCI related to the non-credit portion of other than-temporary impairments on non-agency residential-mortgage-backed securities	0.9
Unrealized losses included in AOCI related to the non-credit portion of other than-temporary impairments on non-agency residential-mortgage-backed securities		(1.9)
Securities held on deposit with various state regulatory authorities, fair value	15,009.3		19.4
Total other-than-temporary impairments	0.1	1.2	7.5
Net Investment Income
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities On Fixed Maturity Under Commitment Agreement	5,033.4	8,986.9	4,603.0
Available-for-sale Securities, Gross Realized Gains	108.5	351.2	295.9
Available-for-sale Securities, Gross Realized Losses	(4.9)	18.3	13.8
Insurance and Financial Services
Schedule of Available-for-sale Securities [Line Items]
Fair value on securities	$ 0.2	$ 60.9
Percentage of amortized cost	20.00%
Percentage of carrying values of all investments (less than 1%)	1.00%