v3.25.1
Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Embedded Derivative in Modified Coinsurance Arrangement    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation    
Derivatives Measured with Unobservable Inputs, Beginning Balance $ 11.5 $ (1.5)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings (1.9) 6.1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss) 0.0 0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases 0.0 0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales 0.0 0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3 0.0 0.0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3 0.0 0.0
Derivatives Measured with Unobservable Inputs, Ending Balance 9.6 4.6
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss), OCI 0.0 0.0
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss) (1.9) 6.1
Public Utility, Bonds    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation    
Assets Measured with Unobservable Inputs, Beginning Balance 0.0  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (1.5)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 1.6  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0.0  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (12.3)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 12.2  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0.0  
Assets Measured with Unobservable Inputs, Ending Balance 0.0  
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI 0.0  
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0.0  
Mortgage/Asset-backed Securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation    
Assets Measured with Unobservable Inputs, Beginning Balance 73.5 32.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0.0 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (0.6) 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 4.8 5.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (0.5) (0.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0.0 0.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0.0 0.0
Assets Measured with Unobservable Inputs, Ending Balance 77.2 38.7
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI (0.6) 0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0.0 0.0
All Other Corporate Bonds    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation    
Assets Measured with Unobservable Inputs, Beginning Balance 71.5 123.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (6.2) (2.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (8.3) 2.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0.0 2.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (70.3) (127.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 83.8 138.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (38.2) (45.8)
Assets Measured with Unobservable Inputs, Ending Balance 32.3 90.2
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI (8.3) 2.1
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0.0 0.0
Fixed Maturity Securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation    
Assets Measured with Unobservable Inputs, Beginning Balance 145.0 156.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings (7.7) (2.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) (7.3) 2.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 4.8 7.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales (83.1) (127.7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 96.0 138.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (38.2) (45.8)
Assets Measured with Unobservable Inputs, Ending Balance 109.5 128.9
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI (8.9) 2.1
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) 0.0 0.0
Perpetual Preferred and Equity Securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation    
Assets Measured with Unobservable Inputs, Beginning Balance 24.4 21.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings 0.7 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) 0.0 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 1.7 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales 0.0 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 0.0 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0.0 0.0
Assets Measured with Unobservable Inputs, Ending Balance 26.8 21.6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI 0.0 0.0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) $ 0.7 $ 0.0