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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, 2022
Dec. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
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 $ 211.5  
Balance at the end of the year 312.1 $ 211.5
Middle market loans    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 65.4  
Balance at the end of the year 106.9 65.4
Equity securities    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 3.6  
Balance at the end of the year 6.6 3.6
Short-term investments    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the end of the year 25.6  
Recurring | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) (4.6) (2.2)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 307.1  
Balance at the end of the year 533.0 307.1
Recurring | Level 3 | Liability    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 0.0  
Liability purchases (17.2)  
Liability transfers 0.0  
Liability sales 0.0  
Liability, increase (decrease) in fv included in net income (14.5)  
Balance at the end of the period (31.7) 0.0
Change in unrealized gains or losses 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 0.0  
Liability purchases (17.2)  
Liability transfers 0.0  
Liability sales 0.0  
Liability, increase (decrease) in fv included in net income (14.5)  
Balance at the end of the period (31.7) 0.0
Change in unrealized gains or losses 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) (0.5) (0.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 211.5  
Balance at the end of the year 312.1 211.5
Recurring | Middle market loans | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) (2.1) (1.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 65.3  
Balance at the end of the year 106.9 65.3
Recurring | Asset-backed | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) (2.0) (0.2)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 26.7  
Balance at the end of the year 66.8 26.7
Recurring | Global corporate securities | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses) 0.0  
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the end of the year 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 3.6  
Balance at the end of the year 6.6 3.6
Recurring | Preference shares | Level 3    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Change in unrealized investment gains (losses)   0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 0.0  
Balance at the end of the year   0.0
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  
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the end of the year 25.6  
Assets | Recurring | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 307.1 299.3
Purchases and issuances 378.2 205.1
Settlements and sales (147.4) (182.0)
Increase (decrease) in fair value included in net income (4.9) 4.2
Balance at the end of the year   307.1
Transfers in/(out) 0.0 (19.5)
Assets | Recurring | Commercial mortgage loans | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 211.5 163.6
Purchases and issuances 215.7 169.9
Settlements and sales (113.1) (122.5)
Increase (decrease) in fair value included in net income (2.0) 0.5
Balance at the end of the year   211.5
Transfers in/(out) 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 65.3 112.1
Purchases and issuances 61.8 5.6
Settlements and sales (19.3) (56.1)
Increase (decrease) in fair value included in net income (0.9) 3.7
Balance at the end of the year   65.3
Transfers in/(out) 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 26.7 18.6
Purchases and issuances 54.5 10.0
Settlements and sales (12.6) (1.8)
Increase (decrease) in fair value included in net income (1.8) (0.1)
Balance at the end of the year   26.7
Transfers in/(out) 0.0 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 0.0  
Purchases and issuances 15.1  
Settlements and sales 0.0  
Increase (decrease) in fair value included in net income (0.1)  
Balance at the end of the year   0.0
Transfers in/(out) 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 3.6 5.0
Purchases and issuances 5.5 0.1
Settlements and sales (2.4) (1.6)
Increase (decrease) in fair value included in net income (0.1) 0.1
Balance at the end of the year   3.6
Transfers in/(out) 0.0 0.0
Assets | Recurring | Preference shares | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year   0.0
Purchases and issuances   19.5
Settlements and sales   0.0
Increase (decrease) in fair value included in net income   0.0
Transfers in/(out)   (19.5)
Assets | Recurring | Short-term investments | Level 3    
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]    
Balance at the beginning of the year 0.0  
Purchases and issuances 25.6  
Settlements and sales 0.0  
Increase (decrease) in fair value included in net income 0.0  
Balance at the end of the year   $ 0.0
Transfers in/(out) $ 0.0