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, 2020	Dec. 31, 2019
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (22.8)	$ (31.1)
Derivative Liability Gain (Loss) Included in Earnings	(17.0)	8.3
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	(39.8)	(22.8)
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)	(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
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	41.8	0.0
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	2.2	2.8
Investment Purchases	0.0	13.3
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	0.0	25.7
Investment Level 3 Transfers Out of	(28.5)	0.0
Assets Measured with Unobservable Inputs, Ending Balance	15.5	41.8
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
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)	1.7
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	21.8	31.4
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	0.0	0.6
Investment Purchases	0.0	0.0
Investment Sales	0.0	(10.2)
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	21.8	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
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)	0.0
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	14.6	84.7
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	3.8	0.8
Investment Purchases	0.0	0.0
Investment Sales	0.0	0.0
Investment Level 3 Transfers Into	175.9	6.0
Investment Level 3 Transfers Out of	(8.6)	(76.9)
Assets Measured with Unobservable Inputs, Ending Balance	185.7	14.6
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
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)	3.7
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	34.1	0.0
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	(3.0)	(2.7)
Investment Purchases	0.0	0.0
Investment Sales	(67.9)	0.0
Investment Level 3 Transfers Into	118.1	36.8
Investment Level 3 Transfers Out of	0.0	0.0
Assets Measured with Unobservable Inputs, Ending Balance	81.3	34.1
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
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)	(3.5)
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	600.5	1,495.8
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	29.8	(3.1)
Investment Purchases	194.7	93.0
Investment Sales	(36.1)	(86.0)
Investment Level 3 Transfers Into	343.1	190.6
Investment Level 3 Transfers Out of	(188.9)	(1,089.8)
Assets Measured with Unobservable Inputs, Ending Balance	943.1	600.5
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
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)	26.7
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0.0	21.1
Investment Gain (Loss) included in Earnings		0.0
Investment Gain (Loss) included in OCI		0.0
Investment Purchases		0.0
Investment Sales		0.0
Investment Level 3 Transfers Into		0.0
Investment Level 3 Transfers Out of		(21.1)
Assets Measured with Unobservable Inputs, Ending Balance		0.0
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	712.8	1,633.0
Investment Gain (Loss) included in Earnings	0.0	0.0
Investment Gain (Loss) included in OCI	32.8	(1.6)
Investment Purchases	194.7	106.3
Investment Sales	(104.0)	(96.2)
Investment Level 3 Transfers Into	637.1	259.1
Investment Level 3 Transfers Out of	(226.0)	(1,187.8)
Assets Measured with Unobservable Inputs, Ending Balance	1,247.4	712.8
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
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)	28.6
Perpetual Preferred Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	4.6	4.6
Investment Gain (Loss) included in Earnings	0.1	0.0
Investment Gain (Loss) included in OCI	0.0	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	4.7	$ 4.6
Perpetual Preferred Equity 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
Perpetual Preferred Equity 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