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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

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