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Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
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     $ 274.9 $ 312.1 $ 312.1  
Balance at the end of the year $ 133.7   133.7   274.9 $ 312.1
Middle market loans            
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     84.8 106.9 106.9  
Balance at the end of the year 64.0   64.0   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 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     18.0 25.6 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) 6.7 $ (3.6) (10.9) (13.4) (15.7) (4.6)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     489.9 533.0 533.0  
Balance at the end of the year 366.5 480.9 366.5 480.9 489.9 533.0
Recurring | Level 3 | Liability            
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year (9.7) (23.4) (16.5) (31.7) (31.7) 0.0
Liability purchases 0.0 0.0 0.0 0.0 0.0 (17.2)
Liability transfers 0.0 0.0 0.0 0.0 0.0 0.0
Liability sales 0.0 0.0 0.0 0.0 0.0 0.0
Liability, increase (decrease) in fv included in net income 5.1 0.0 11.9 8.3 15.2 (14.5)
Balance at the end of the period (4.6) (23.4) (4.6) (23.4) (16.5) (31.7)
Change in unrealized gains or losses 5.1 0.0 11.9 8.3 15.2 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 (9.7) (23.4) (16.5) (31.7) (31.7) 0.0
Liability purchases 0.0 0.0 0.0 0.0 0.0 (17.2)
Liability transfers 0.0 0.0 0.0 0.0 0.0 0.0
Liability sales 0.0 0.0 0.0 0.0 0.0 0.0
Liability, increase (decrease) in fv included in net income 5.1 0.0 11.9 8.3 15.2 (14.5)
Balance at the end of the period (4.6) (23.4) (4.6) (23.4) (16.5) (31.7)
Change in unrealized gains or losses 5.1 0.0 11.9 8.3 15.2 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) 5.7 (3.7) (12.0) (14.1) (17.9) (0.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     274.9 312.1 312.1  
Balance at the end of the year   288.1   288.1 274.9 312.1
Recurring | Middle market loans | Level 3            
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]            
Change in unrealized investment gains (losses) 0.3 0.0 0.3 0.2 0.5 (2.1)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     84.8 106.9 106.9  
Balance at the end of the year   84.8   84.8 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) 0.5 0.0 0.4 0.4 1.7 (2.0)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     82.9 66.8 66.8  
Balance at the end of the year   60.6   60.6 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) 0.3 (0.2) 0.2 (0.2) (0.2) 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     14.4 15.0 15.0  
Balance at the end of the year   14.4   14.4 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)   0.3   0.3 0.0 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     0.0 6.6 6.6  
Balance at the end of the year   14.8   14.8 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 0.0 0.0 0.0 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward]            
Balance at the beginning of the year     18.0 25.6 25.6  
Balance at the end of the year   18.2   18.2 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 474.2 482.4 489.9 533.0 533.0 307.1
Purchases and issuances 10.4 79.3 64.8 102.3 113.7 378.2
Settlements and sales (114.5) (76.4) (166.9) (134.8) (137.2) (147.4)
Increase (decrease) in fair value included in net income (3.6) (4.4) (21.3) (12.9) (12.8) (4.9)
Balance at the end of the year         489.9 533.0
Transfers in/(out) 0.0 0.0 0.0 0.0 5.3 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 240.4 295.0 274.9 312.1 312.1 211.5
Purchases and issuances 0.0 23.5 0.4 39.4 40.6 215.7
Settlements and sales (101.6) (26.0) (118.8) (49.2) (61.5) (113.1)
Increase (decrease) in fair value included in net income (5.1) (4.4) (22.8) (14.2) (16.3) (2.0)
Balance at the end of the year 133.7   133.7   274.9 312.1
Transfers in/(out) 0.0 0.0 0.0 0.0 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 57.3 102.1 84.8 106.9 106.9 65.3
Purchases and issuances 0.3 4.0 0.3 4.1 18.3 61.8
Settlements and sales (4.8) (21.3) (32.6) (26.7) (41.9) (19.3)
Increase (decrease) in fair value included in net income 0.3 0.0 0.6 0.5 1.5 (0.9)
Balance at the end of the year 64.0   64.0   84.8 106.9
Transfers in/(out) 0.0 0.0 0.0 0.0 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 133.5 58.4 82.9 66.8 66.8 26.7
Purchases and issuances 0.1 31.2 54.1 31.2 21.9 54.5
Settlements and sales (4.8) (29.0) (8.1) (32.8) (7.6) (12.6)
Increase (decrease) in fair value included in net income 0.4 0.0 0.3 0.9 2.0 (1.8)
Balance at the end of the year 129.2   129.2   82.9 66.8
Transfers in/(out) 0.0 0.0 0.0 0.0 5.3 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 14.1 14.8 14.4 15.0 15.0 0.0
Purchases and issuances 0.0 0.0 0.0 0.0 0.0 15.1
Settlements and sales (0.1) (0.1) (0.3) (0.3) (0.4) 0.0
Increase (decrease) in fair value included in net income 0.3 (0.3) 0.2 (0.3) (0.2) (0.1)
Balance at the end of the year 14.3   14.3   14.4 15.0
Transfers in/(out) 0.0 0.0 0.0 0.0 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   12.1   6.6 6.6 3.6
Purchases and issuances   2.4   9.4 0.0 5.5
Settlements and sales   0.0   (0.2) 0.0 (2.4)
Increase (decrease) in fair value included in net income   0.3   0.2 0.0 (0.1)
Balance at the end of the year           6.6
Transfers in/(out)   0.0   0.0 0.0 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 14.1 0.0 18.0 25.6 25.6 0.0
Purchases and issuances 0.0 18.2 0.0 18.2 18.2 25.6
Settlements and sales (3.2) 0.0 (7.1) (25.6) (25.8) 0.0
Increase (decrease) in fair value included in net income 0.0 0.0 0.0 0.0 0.0 0.0
Balance at the end of the year 0.0   0.0   18.0 25.6
Transfers in/(out) $ 0.0 $ 0.0 $ 0.0 $ 0.0 $ 0.0 $ 0.0