Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Mar. 31, 2013	Mar. 31, 2014 Investment industry	Mar. 31, 2013	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Change Of Intent Operations	$ 0		$ 0.7
Document Period End Date		Mar. 31, 2014
Available For Sale Securities Gross Unrealized Losses Investment		138.4		263.9
Other than temporary impairment credit losses recognized in earnings additions other	0.4		0.9
Other Than Temporary Impairment Credit Losses Recognized In Earnings Additions Including Credit Impairments	0		0.3
Amortized cost on other-than temporary impairments				2.7
Fair value on other than temporary impairments				1.8
Significant investment, number of industries		1
Industry holdings, number of top investments		10
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments		37.70%
Total investments, Carrying Value		18,312.8		16,465.9
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Losses Investment		8.5		5.2
Non-agency residential mortgage-backed securities [Member]
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)
Available For Sale Securities Gross Unrealized Losses Investment		10.1		13.4
Other Marketable Securities, Current		14,334.4		19.4
FGL [Member] | Stockholders' Equity, Total [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of investment		1,851.3		1,983.7
Percentage of fair value of the invested assets portfolio		10.10%		12.00%
Investment in issuers		18		19
Percentage of stockholders equity		10.00%
Net Investment Income [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Losses		(2.5)	(0.6)
Available for sale securities on fixed maturity		2,627.9	5,738.3
Available-for-sale Securities, Gross Realized Gains		22.2	249.0
Concentration [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Securities Held by Subsidiaries Subject to Specialized Industry Accounting Principles at Fair Value		2,019.4		1,892.1
Percentage of fair value of the invested assets portfolio		11.00%		11.50%
Number Of Issuers In Investment		78
Concentration [Member] | FGL [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value of investment		250.0		150.7
Percentage of fair value of the invested assets portfolio		1.40%		0.90%
Insurance and Financial Services [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value on securities		1.0		60.9
Percentage of amortized cost		20.00%
Percentage of carrying values of all investments (less than 1%)		1.00%
FHLB of Atlanta Funding Agreement Collateral Held [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Owned, at Fair Value		592.3		604.9
Fixed Maturities Available For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Cost or Amortized Cost		16,040.5		15,062.1
Available For Sale Securities Gross Unrealized Losses Investment		(130.3)		(253.6)
Total investments, Carrying Value		16,626.6		15,300.0
Fixed Maturities Available For Sale [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Cost or Amortized Cost		1,576.6		1,505.7
Available For Sale Securities Gross Unrealized Losses Investment		(8.5)		(5.2)
Total investments, Carrying Value		1,587.2		1,523.1
Fixed Maturities Available For Sale [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Cost or Amortized Cost		1,730.9		1,304.0
Available For Sale Securities Gross Unrealized Losses Investment		(10.1)		(13.4)
Total investments, Carrying Value		$ 1,847.1		$ 1,368.0