Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value on Non-Recurring Basis (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Sep. 30, 2015		Dec. 31, 2014	Sep. 30, 2014
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 73.9	$ 137.8	$ 73.9	$ 137.8	$ 74.8	$ 101.3		$ 122.5	$ 117.2
Assets
Cash and cash equivalents		465.6	430.3	465.6	430.3		695.2			742.8
Fixed maturity securities, available-for-sale:
Asset-backed loans		136.8		136.8			226.7
Liabilities
Debt		6,232.0		6,232.0			6,310.5	[1],[2]
Derivatives, Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(0.3)	5.7	0.5	5.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)					(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		3.0	12.0	8.1	22.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(3.1)	(2.4)	(30.0)	(6.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(0.5)		(6.0)
Funds Withheld Receivable		1,679.4		1,679.4			1,710.1
Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			15.8		15.8		14.1		15.8	16.3
Derivatives, Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)					(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases				0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales				(13.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements				0.0
Other Invested Assets [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0.0		0.0			2.8
Derivatives, Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				2.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases				0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales				0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements				(5.5)
Net transfer In (Out) of Level 3 (a)				0.0
Level 2 [Member]
Liabilities
Total debt	[1],[2]	6,391.0		6,391.0			6,398.0
Level 3 [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[1]	2.5		2.5			2.5
Asset-backed loans	[1]	136.8		136.8			226.7
Liabilities
Total debt	[1],[2]	80.7		80.7			99.1
Derivatives, Liabilities
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	895.9		895.9			960.3
Recurring [Member]
Derivatives, Liabilities
Liabilities, Fair Value Disclosure, Recurring		799.9		799.9			781.3
Funds Withheld Receivable		1,679.4		1,679.4			1,710.1
Assets, Fair Value Disclosure, Recurring		1,694.6		1,694.6			1,780.1
Recurring [Member] | Foreign Exchange Forward [Member]
Derivatives, Liabilities
Derivative Liability		5.3		5.3			1.6
Recurring [Member] | Commodity Option [Member]
Derivatives, Assets
Derivatives		1.7		1.7			7.9
Derivatives, Liabilities
Derivative Liability		2.0		2.0			5.6
Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Assets
Derivatives		10.1		10.1			6.4
Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability		2.1		2.1			2.6
Recurring [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities		0.0		0.0			14.1
Recurring [Member] | Foreign Exchange Contract [Member] | Foreign Exchange Forward [Member]
Derivatives, Assets
Derivatives		3.4		3.4			6.0
Recurring [Member] | Equity [Member]
Fixed maturity securities, available-for-sale:
Trading Securities		0.0		0.0			32.8
Recurring [Member] | Other Invested Assets [Member]
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		0.0			2.8
Recurring [Member] | Level 1 [Member]
Fixed maturity securities, available-for-sale:
Total financial assets		0.0		0.0			0.0
Derivatives, Liabilities
Financial Liabilities Fair Value Disclosure		0.0		0.0			0.0
Liabilities, Fair Value Disclosure, Recurring		0.0		0.0			0.0
Funds Withheld Receivable		73.9		73.9			45.8
Assets, Fair Value Disclosure, Recurring		73.9		73.9			78.6
Recurring [Member] | Level 1 [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative Liability		136.5		136.5			142.3
Recurring [Member] | Level 1 [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability							0.0
Recurring [Member] | Level 1 [Member] | Interest Rate Contract [Member]
Derivatives, Assets
Derivatives		0.0		0.0			0.0
Recurring [Member] | Level 1 [Member] | Equity [Member]
Fixed maturity securities, available-for-sale:
Trading Securities		0.0		0.0			32.8
Recurring [Member] | Level 1 [Member] | Other Invested Assets [Member]
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		0.0			0.0
Recurring [Member] | Level 2 [Member]
Fixed maturity securities, available-for-sale:
Total financial assets		0.0		0.0			0.0
Derivatives, Liabilities
Financial Liabilities Fair Value Disclosure		6,391.0		6,391.0			6,398.0
Liabilities, Fair Value Disclosure, Recurring		9.4		9.4			9.8
Funds Withheld Receivable		1,531.6		1,531.6			1,579.9
Assets, Fair Value Disclosure, Recurring		1,546.8		1,546.8			1,600.2
Recurring [Member] | Level 2 [Member] | Foreign Exchange Forward [Member]
Derivatives, Liabilities
Derivative Liability		5.3		5.3			1.6
Recurring [Member] | Level 2 [Member] | Commodity Option [Member]
Derivatives, Assets
Derivatives		1.7		1.7			7.9
Derivatives, Liabilities
Derivative Liability		2.0		2.0			5.6
Recurring [Member] | Level 2 [Member] | Interest Rate Contract [Member]
Derivatives, Assets
Derivatives		10.1		10.1			6.4
Recurring [Member] | Level 2 [Member] | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability		2.1		2.1			2.6
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities		0.0		0.0			0.0
Recurring [Member] | Level 2 [Member] | Foreign Exchange Contract [Member] | Foreign Exchange Forward [Member]
Derivatives, Assets
Derivatives		3.4		3.4			6.0
Recurring [Member] | Level 3 [Member]
Fixed maturity securities, available-for-sale:
Total financial assets		139.3		139.3			229.2
Derivatives, Liabilities
Financial Liabilities Fair Value Disclosure		976.6		976.6			1,059.4
Liabilities, Fair Value Disclosure, Recurring		790.5		790.5			771.5
Funds Withheld Receivable		73.9		73.9			84.4
Assets, Fair Value Disclosure, Recurring		73.9		73.9			101.3
Recurring [Member] | Level 3 [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative Liability		136.5		136.5			142.3
Recurring [Member] | Level 3 [Member] | Interest Rate Contract [Member]
Derivatives, Assets
Derivatives		0.0		0.0			0.0
Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities		0.0		0.0			14.1
Recurring [Member] | Level 3 [Member] | Foreign Exchange Contract [Member] | Foreign Exchange Forward [Member]
Derivatives, Assets
Derivatives							0.0
Recurring [Member] | Level 3 [Member] | Other Invested Assets [Member] | Other Invested Assets [Member]
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		0.0
Contingent Purchase Price Reduction Receivable [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			47.0		47.0				41.5	41.5
Derivatives, Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings			5.5		5.5
Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		654.0	584.5	654.0	584.5	629.0	629.2		496.4	151.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		23.9	22.4	20.2	22.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases			73.6		420.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		1.1	7.9	4.6	9.9
Equity conversion feature of preferred stock [Member]
Derivatives, Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		790.5	740.4	790.5	740.4	$ 768.9	771.5		$ 653.4	$ 302.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		20.5	21.3	14.4	27.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0.0	73.6	0.0	420.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales		0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		1.1	7.9	4.6	9.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0.0	$ 0.0	0.0	$ 0.0
Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[1]	2.5		2.5			2.5
Asset-backed loans	[1]	136.8		136.8			226.7
Liabilities
Total debt	[1],[2]	6,471.7		6,471.7			6,497.1
Derivatives, Liabilities
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	895.9		895.9			960.3
Estimate of Fair Value Measurement [Member] | Recurring [Member]
Fixed maturity securities, available-for-sale:
Total financial assets		139.3		139.3			229.2
Derivatives, Liabilities
Financial Liabilities Fair Value Disclosure		7,367.6		7,367.6			7,457.4
Estimate of Fair Value Measurement [Member] | Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability		654.0		654.0			629.2
Reported Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[1]	2.5		2.5			2.5
Asset-backed loans	[1]	136.8		136.8			226.7
Total financial assets		139.3		139.3
Liabilities
Total debt	[1],[2]	6,232.0		6,232.0
Derivatives, Liabilities
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	1,034.3		1,034.3			1,084.5
Financial Liabilities Fair Value Disclosure		7,266.3		7,266.3
Reported Value Measurement [Member] | Recurring [Member]
Fixed maturity securities, available-for-sale:
Total financial assets							229.2
Derivatives, Liabilities
Financial Liabilities Fair Value Disclosure							7,395.0
Option Pricing [Member] | Recurring [Member] | Level 3 [Member] | Other Invested Assets [Member] | Other Invested Assets [Member]
Fixed maturity securities, available-for-sale:
Other invested assets							2.8
Discounted Cash Flow [Member] | Recurring [Member] | Level 3 [Member]
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		0.0			2.8
Derivatives, Liabilities
Funds Withheld Receivable		15.1		15.1			19.5
Discounted Cash Flow [Member] | Recurring [Member] | Level 3 [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability		$ 654.0		$ 654.0			$ 629.2

[1] The carrying amounts of cash and cash equivalents, trade receivables, accounts payable and accrued investment income approximate fair value due to their short duration and, accordingly, they are not presented in the tables above.

[2] The fair values of debt set forth above are generally based on quoted or observed market prices.