Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 74.8	$ 122.5	$ 101.3	$ 117.2
Total Gains (Losses) Included in Earnings	(0.3)	5.7	0.5	5.6
Total Gains (Losses) Included in AOCI				(0.5)
Purchases	3.0	12.0	8.1	22.0
Sales	(3.1)	(2.4)	(30.0)	(6.5)
Settlements	(0.5)		(6.0)
Balance at End of Period	73.9	137.8	73.9	137.8
Front Street Future Policyholder Benefits Reserve [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	629.0	496.4	629.2	151.3
Total Gains (Losses) Included in Earnings	23.9	22.4	20.2	22.6
Purchases		73.6		420.5
Settlements	(1.1)	(7.9)	(4.6)	(9.9)
Balance at End of Period	654.0	584.5	654.0	584.5
Fixed Index Annuity [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	139.9	157.0	142.3	150.8
Total Gains (Losses) Included in Earnings	(3.4)	(1.1)	(5.8)	5.1
Balance at End of Period	136.5	155.9	136.5	155.9
Equity conversion feature of preferred stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	768.9	653.4	771.5	302.1
Total Gains (Losses) Included in Earnings	20.5	21.3	14.4	27.7
Total Gains (Losses) Included in AOCI	0.0	0.0	0.0	0.0
Purchases	0.0	73.6	0.0	420.5
Sales	0.0	0.0	0.0	0.0
Settlements	(1.1)	(7.9)	(4.6)	(9.9)
Net transfer In (Out) of Level 3	0.0	0.0	0.0	0.0
Balance at End of Period	790.5	740.4	790.5	740.4
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		15.8	14.1	16.3
Total Gains (Losses) Included in Earnings			(0.5)
Total Gains (Losses) Included in AOCI				(0.5)
Purchases			0.0
Sales			(13.6)
Settlements			0.0
Balance at End of Period		15.8		15.8
Funds Withheld Receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		65.2		59.4
Total Gains (Losses) Included in Earnings		0.2		0.1
Purchases		12.0		22.0
Sales		(2.4)		(6.5)
Balance at End of Period		75.0		75.0
Other Invested Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period			2.8
Total Gains (Losses) Included in Earnings			2.7
Total Gains (Losses) Included in AOCI			0.0
Purchases			0.0
Sales			0.0
Settlements			(5.5)
Net transfer In (Out) of Level 3 (a)			0.0
Balance at End of Period	0.0		0.0
Funds withheld for reinsurance liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	74.8		84.4
Total Gains (Losses) Included in Earnings	(0.3)		(1.7)
Total Gains (Losses) Included in AOCI	0.0		0.0
Purchases	3.0		8.1
Sales	(3.1)		(16.4)
Settlements	(0.5)		(0.5)
Net transfer In (Out) of Level 3 (a)	0.0		0.0
Balance at End of Period	$ 73.9		$ 73.9
Contingent Purchase Price Reduction Receivable [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		41.5		41.5
Total Gains (Losses) Included in Earnings		5.5		5.5
Balance at End of Period		$ 47.0		$ 47.0