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Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2023 | 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 | $ 312.1 | $ 211.5 |
Balance at the end of the year | 295.0 | 312.1 | $ 211.5
Middle market loans |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 106.9 | 65.4 |
Balance at the end of the year | 102.1 | 106.9 | 65.4
Equity securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 6.6 | 3.6 |
Balance at the end of the year | 12.1 | 6.6 | 3.6
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 |  | 25.6 |
Recurring | Level 3 |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |
Change in unrealized investment gains (losses) | (13.7) | (4.6) | (2.2)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 533.0 | 307.1 |
Balance at the end of the year | 482.4 | 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 | (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 | 8.3 | (14.5) |
Balance at the end of the period | (23.4) | (31.7) | 0.0
Change in unrealized gains or losses | 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 |  | (14.5) |
Balance at the end of the period | (23.4) | (31.7) | 0.0
Change in unrealized gains or losses | 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) | (10.5) | (0.5) | (0.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 312.1 | 211.5 |
Balance at the end of the year | 295.0 | 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) | (2.0) | (2.1) | (1.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 106.9 | 65.3 |
Balance at the end of the year | 102.1 | 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) | (1.1) | (2.0) | (0.2)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 66.8 | 26.7 |
Balance at the end of the year | 58.4 | 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) | (0.1) | 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 | 14.8 | 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 | 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Balance at the beginning of the year | 6.6 | 3.6 |
Balance at the end of the year | 12.1 | 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 | 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 | 0.0 | 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 | 299.3
Purchases and issuances | 23.0 | 378.2 | 205.1
Settlements and sales | (58.4) | (147.4) | (182.0)
Increase (decrease) in fair value included in net income | (8.5) | (4.9) | 4.2
Balance at the end of the year |  | 533.0 | 307.1
Transfers in/(out) | (6.7) | 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 | 312.1 | 211.5 | 163.6
Purchases and issuances | 15.9 | 215.7 | 169.9
Settlements and sales | (23.2) | (113.1) | (122.5)
Increase (decrease) in fair value included in net income | (9.8) | (2.0) | 0.5
Balance at the end of the year |  | 312.1 | 211.5
Transfers in/(out) | 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 | 106.9 | 65.3 | 112.1
Purchases and issuances | 0.1 | 61.8 | 5.6
Settlements and sales | (5.4) | (19.3) | (56.1)
Increase (decrease) in fair value included in net income | 0.5 | (0.9) | 3.7
Balance at the end of the year |  | 106.9 | 65.3
Transfers in/(out) | 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 | 66.8 | 26.7 | 18.6
Purchases and issuances | 0.0 | 54.5 | 10.0
Settlements and sales | (3.8) | (12.6) | (1.8)
Increase (decrease) in fair value included in net income | 0.9 | (1.8) | (0.1)
Balance at the end of the year |  | 66.8 | 26.7
Transfers in/(out) | (5.5) | 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 | 15.0 | 0.0 |
Purchases and issuances | 0.0 | 15.1 |
Settlements and sales | (0.2) | 0.0 |
Increase (decrease) in fair value included in net income | 0.0 | (0.1) |
Balance at the end of the year |  | 15.0 | 0.0
Transfers in/(out) | 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 | 5.0
Purchases and issuances | 7.0 | 5.5 | 0.1
Settlements and sales | (0.2) | (2.4) | (1.6)
Increase (decrease) in fair value included in net income | (0.1) | (0.1) | 0.1
Balance at the end of the year |  | 6.6 | 3.6
Transfers in/(out) | (1.2) | 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) |  | 0.0 | (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 | 25.6 | 0.0 |
Purchases and issuances | 0.0 | 25.6 |
Settlements and sales | (25.6) | 0.0 |
Increase (decrease) in fair value included in net income | 0.0 | 0.0 |
Balance at the end of the year |  | 25.6 | $ 0.0
Transfers in/(out) | $ 0.0 | $ 0.0 |

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