Fair Value of Financial Instruments - Changes in Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2017		Mar. 31, 2016	Mar. 31, 2017		Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 46.4		$ 65.1	$ 78.1		$ 91.6
Total Gains (Losses) Included in Earnings			(0.3)	(1.3)		0.3
Purchases	6.1		3.1	9.1		8.1
Sales	(1.0)		(3.4)	(7.3)		(30.0)
Settlements						(5.5)
Net transfer In (Out) of Level 3 (a)	(2.4)	[1]		(29.5)	[2]
Balance at End of Period	49.1		64.5	49.1		64.5
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	755.7		768.9	763.0		771.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	20.9		20.5	(1.7)		14.4
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
Purchases	0.0		0.0	0.0		0.0
Sales	0.0		0.0	0.0		0.0
Settlements	(5.6)		(1.1)	(20.9)		4.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	0.0	[1]	0.0	0.0	[2]	0.0
Balance at End of Period	782.2		790.5	782.2		790.5
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	634.5		629.0	631.8		629.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	21.1		23.9	8.5		20.2
Settlements	(5.6)		(1.1)	(20.9)		4.6
Balance at End of Period	661.2		654.0	661.2		654.0
Fixed Index Annuity [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	121.2		139.9	131.2		142.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(0.2)		(3.4)	(10.2)		(5.8)
Balance at End of Period	121.0		136.5	121.0		136.5
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						(1.9)
Purchases						8.1
Sales						(16.4)
Balance at End of Period			64.5			64.5
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	46.4		65.1	78.1
Total Gains (Losses) Included in Earnings	0.0		(0.3)	(1.3)
Total Gains (Losses) Included in AOCI	0.0		0.0	0.0
Purchases	6.1		3.1	9.1
Sales	(1.0)		(3.4)	(7.3)
Settlements	0.0		0.0	0.0
Net transfer In (Out) of Level 3 (a)	(2.4)	[1]	0.0	(29.5)	[2]
Balance at End of Period	$ 49.1		$ 64.5	$ 49.1		$ 64.5

[1] (a) During the three months ended March 31, 2017, the net transfer out of Level 3 was exclusively to Level 2.

[2] (a) During the six months ended March 31, 2017, the net transfer out of Level 3 was exclusively to Level 2.