Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 101.3	$ 117.2
Total Gains (Losses) Included in Earnings	0.8	(0.1)
Total Gains (Losses) Included in AOCI		(0.5)
Purchases	5.1	10.0
Sales	(26.9)	(4.1)
Settlements	(5.5)
Balance at End of Period	74.8	122.5
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	629.2	151.3
Total Gains (Losses) Included in Earnings	(3.7)	0.2
Purchases		346.9
Settlements	(3.5)	(2.0)
Balance at End of Period	629.0	496.4
Fixed Index Annuity [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	142.3	150.8
Total Gains (Losses) Included in Earnings	(2.4)	6.2
Balance at End of Period	139.9	157.0
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	771.5	302.1
Total Gains (Losses) Included in Earnings	(6.1)	6.4
Total Gains (Losses) Included in AOCI	0.0	0.0
Purchases	0.0	346.9
Sales	0.0	0.0
Settlements	(3.5)	(2.0)
Net transfer In (Out) of Level 3	0.0	0.0
Balance at End of Period	768.9	653.4
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	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
Funds Withheld Receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		59.4
Total Gains (Losses) Included in Earnings		(0.1)
Purchases		10.0
Sales		(4.1)
Balance at End of Period		65.2
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
Funds withheld for reinsurance liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	84.4
Total Gains (Losses) Included in Earnings	(1.4)
Total Gains (Losses) Included in AOCI	0.0
Purchases	5.1
Sales	(13.3)
Settlements	0.0
Net transfer In (Out) of Level 3 (a)	0.0
Balance at End of Period	$ 74.8
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
Balance at End of Period		$ 41.5