Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (39.8)	$ (22.8)
Derivative Liability Gain (Loss) Included in Earnings	9.7	(17.0)
Derivative Liability Gain (Loss) Included in OCI	0.0	0.0
Derivative Purchases	0.0	0.0
Derivative Sales	0.0	0.0
Derivative Transfers Into Level 3	0.0	0.0
Derivative Transfers Out of Level 3	0.0	0.0
Derivatives Measured with Unobservable Inputs, Ending Balance	(30.1)	(39.8)
Embedded Derivative in Modified Coinsurance Arrangement | Net Investment Income
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss)	9.7	(17.0)
Embedded Derivative in Modified Coinsurance Arrangement | Other Comprehensive Income (Loss)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	15.5	41.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(2.1)	2.2
Investment Purchases	0.0	0.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0
Investment Level 3 Transfers Out of	0.0	(28.5)
Assets Measured with Unobservable Inputs, Ending Balance	13.4	15.5
States, Municipalities, and Political Subdivisions | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
States, Municipalities, and Political Subdivisions | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(2.1)	1.7
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	21.8	21.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(1.0)	0.0
Investment Purchases	0.0	0.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	20.8	21.8
Foreign Governments | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Foreign Governments | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(1.0)	0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	185.7	14.6
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(2.3)	3.8
Investment Purchases	0.0	0.0
Investment Sales	(44.0)	0.0
Investment Level 3 Transfers Into	36.0	175.9
Investment Level 3 Transfers Out of	(130.9)	(8.6)
Assets Measured with Unobservable Inputs, Ending Balance	44.5	185.7
Public Utilities | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Public Utilities | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(2.3)	3.7
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	81.3	34.1
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(96.0)	(3.0)
Investment Purchases	0.0	0.0
Investment Sales	(72.3)	(67.9)
Investment Level 3 Transfers Into	274.2	118.1
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	187.2	81.3
Mortgage/Asset-backed Securities | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Mortgage/Asset-backed Securities | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(96.0)	(3.5)
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	943.1	600.5
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(24.6)	29.8
Investment Purchases	249.9	194.7
Investment Sales	(80.2)	(36.1)
Investment Level 3 Transfers Into	77.6	343.1
Investment Level 3 Transfers Out of	(304.3)	(188.9)
Assets Measured with Unobservable Inputs, Ending Balance	861.5	943.1
All Other Corporate Bonds | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
All Other Corporate Bonds | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(24.6)	26.7
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,247.4	712.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(126.0)	32.8
Investment Purchases	249.9	194.7
Investment Sales	(196.5)	(104.0)
Investment Level 3 Transfers Into	387.8	637.1
Investment Level 3 Transfers Out of	435.2	226.0
Assets Measured with Unobservable Inputs, Ending Balance	1,127.4	1,247.4
Fixed Maturity Securities | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.0	0.0
Fixed Maturity Securities | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(126.0)	28.6
Perpetual Preferred Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	4.7	4.6
Investment Gain (Loss) included in Earnings	0.1	0.1
Investment Gain (Loss) included in OCI	0.0	0.0
Investment Purchases	1.0	0.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	0.0
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	5.8	4.7
Perpetual Preferred Securities | Net Investment Income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0.1	0.1
Perpetual Preferred Securities | Other Comprehensive Income (Loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ 0.0	$ 0.0