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Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Beginning of the year $ 3,788.6  
End of the year $ 4,122.6 $ 3,788.6
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] Accumulated other comprehensive (loss)  
Commercial mortgage loans    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year $ 312.1  
Balance at the end of the year 274.9 312.1
Middle market loans    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 106.9  
Balance at the end of the year 84.8 106.9
Equity securities    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 6.6  
Balance at the end of the year   6.6
Short-term investments    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 25.6  
Balance at the end of the year 18.0 25.6
Recurring | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) (15.7) (4.6)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 533.0  
Balance at the end of the year 489.9 533.0
Recurring | Level 3 | Liability    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year (31.7) 0.0
Liability purchases 0.0 (17.2)
Liability transfers 0.0 0.0
Liability sales 0.0 0.0
Liability, increase (decrease) in fv included in net income 15.2 (14.5)
Balance at the end of the period (16.5) (31.7)
Change in unrealized gains or losses 15.2 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 0.0 0.0
Recurring | Level 3 | Liability | Loss Portfolio Transfer    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year (31.7) 0.0
Liability purchases 0.0 (17.2)
Liability transfers 0.0 0.0
Liability sales 0.0 0.0
Liability, increase (decrease) in fv included in net income 15.2 (14.5)
Balance at the end of the period (16.5) (31.7)
Change in unrealized gains or losses 15.2 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 0.0 0.0
Recurring | Commercial mortgage loans | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) (17.9) (0.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 312.1  
Balance at the end of the year 274.9 312.1
Recurring | Middle market loans | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) 0.5 (2.1)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 106.9  
Balance at the end of the year 84.8 106.9
Recurring | Asset-backed | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) 1.7 (2.0)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 66.8  
Balance at the end of the year 82.9 66.8
Recurring | Global corporate securities | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) (0.2) 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 15.0  
Balance at the end of the year 14.4 15.0
Recurring | Equity securities | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) 0.0 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 6.6  
Balance at the end of the year 0.0 6.6
Recurring | Short-term investments | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) 0.0 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 25.6  
Balance at the end of the year 18.0 25.6
Assets | Recurring | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 533.0 307.1
Purchases and issuances 113.7 378.2
Transfers in/(out) 5.3 0.0
Settlements and sales (137.2) (147.4)
Increase (decrease) in fair value included in net income (12.8) (4.9)
Balance at the end of the year   533.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (12.1) 0.0
Assets | Recurring | Commercial mortgage loans | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 312.1 211.5
Purchases and issuances 40.6 215.7
Transfers in/(out) 0.0 0.0
Settlements and sales (61.5) (113.1)
Increase (decrease) in fair value included in net income (16.3) (2.0)
Balance at the end of the year   312.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0.0 0.0
Assets | Recurring | Middle market loans | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 106.9 65.3
Purchases and issuances 18.3 61.8
Transfers in/(out) 0.0 0.0
Settlements and sales (41.9) (19.3)
Increase (decrease) in fair value included in net income 1.5 (0.9)
Balance at the end of the year   106.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0.0 0.0
Assets | Recurring | Asset-backed | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 66.8 26.7
Purchases and issuances 21.9 54.5
Transfers in/(out) 5.3 0.0
Settlements and sales (7.6) (12.6)
Increase (decrease) in fair value included in net income 2.0 (1.8)
Balance at the end of the year   66.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (5.5) 0.0
Assets | Recurring | Global corporate securities | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 15.0 0.0
Purchases and issuances 0.0 15.1
Transfers in/(out) 0.0 0.0
Settlements and sales (0.4) 0.0
Increase (decrease) in fair value included in net income (0.2) (0.1)
Balance at the end of the year   15.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 0.0 0.0
Assets | Recurring | Equity securities | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 6.6 3.6
Purchases and issuances 0.0 5.5
Transfers in/(out) 0.0 0.0
Settlements and sales 0.0 (2.4)
Increase (decrease) in fair value included in net income 0.0 (0.1)
Balance at the end of the year   6.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 (6.6) 0.0
Assets | Recurring | Short-term investments | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 25.6 0.0
Purchases and issuances 18.2 25.6
Transfers in/(out) 0.0 0.0
Settlements and sales (25.8) 0.0
Increase (decrease) in fair value included in net income 0.0 0.0
Balance at the end of the year   25.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 $ 0.0 $ 0.0