Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	$ 755.7		$ 763.0
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	78.1	$ 91.6
Total Gains (Losses) Included in Earnings	(1.3)	0.6
Purchases	3.0	5.0
Sales	(6.3)	(26.6)
Settlements		(5.5)
Net transfer In (Out) of Level 3 (a)	(27.1)
Balance at End of Period	46.4	65.1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		771.5
Total Gains (Losses) Included in Earnings	(22.6)	(6.1)
Total Gains (Losses) Included in AOCI	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Settlements	(15.3)	3.5
Net transfer In (Out) of Level 3	0.0	0.0
Balance at End of Period	755.7	768.9
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	631.8	629.2
Total Gains (Losses) Included in Earnings	(12.6)	(3.7)
Settlements	(15.3)	3.5
Balance at End of Period	634.5	629.0
Fixed Index Annuity [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	131.2	142.3
Total Gains (Losses) Included in Earnings	(10.0)	(2.4)
Balance at End of Period	121.2	139.9
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		14.1
Total Gains (Losses) Included in Earnings		(0.5)
Sales		(13.6)
Funds Withheld Receivables [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		74.7
Total Gains (Losses) Included in Earnings		(1.6)
Purchases		5.0
Sales		(13.0)
Balance at End of Period		65.1
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	78.1
Total Gains (Losses) Included in Earnings	(1.3)
Total Gains (Losses) Included in AOCI	0.0
Purchases	3.0
Sales	(6.3)
Settlements	0.0
Net transfer In (Out) of Level 3 (a)	(27.1)
Balance at End of Period	$ 46.4