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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, 2021 | Dec. 31, 2020
Commercial mortgage loans |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | $ 163.6 |
Balance at the end of the year | 211.5 | $ 163.6
Middle market loans |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 112.1 |
Balance at the end of the year | 65.4 | 112.1
Equity securities |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 5.0 |
Balance at the end of the year | 3.6 | 5.0
Recurring | Level 3 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Change in unrealized investment gains (losses) | (2.2) | (5.2)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 299.3 |
Balance at the end of the year | 307.1 | 299.3
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) | 0.1
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 163.6 |
Balance at the end of the year | 211.5 | 163.6
Recurring | Middle market loans | Level 3 |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |
Change in unrealized investment gains (losses) | (1.5) | (5.3)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 112.1 |
Balance at the end of the year | 65.3 | 112.1
Recurring | Asset-backed | 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 | 18.6 |
Balance at the end of the year | 26.7 | 18.6
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 | 5.0 |
Balance at the end of the year | 3.6 | 5.0
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 end of the year | 0.0 |
Assets | Recurring | Level 3 |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 299.3 | 279.5
Purchases and issuances | 205.1 | 93.0
Settlements and sales | (182.2) | (64.3)
Increase (decrease) in fair value included in net income | 4.2 | (17.2)
Balance at the end of the year |  | 299.3
Transfers in/(out) | (19.5) | 8.3
Assets | Recurring | Commercial mortgage loans | Level 3 |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 163.6 | 156.4
Purchases and issuances | 169.9 | 79.7
Settlements and sales | (122.5) | (59.4)
Increase (decrease) in fair value included in net income | 0.5 | (13.1)
Balance at the end of the year |  | 163.6
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 | 112.1 | 111.7
Purchases and issuances | 5.6 | 0.9
Settlements and sales | (56.1) | (4.8)
Increase (decrease) in fair value included in net income | 3.7 | (4.0)
Balance at the end of the year |  | 112.1
Transfers in/(out) | 0.0 | 8.3
Assets | Recurring | Asset-backed | Level 3 |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 18.6 | 8.7
Purchases and issuances | 10.0 | 10.0
Settlements and sales | (1.8) | 0.0
Increase (decrease) in fair value included in net income | (0.1) | (0.1)
Balance at the end of the year |  | 18.6
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 | 5.0 | 2.7
Purchases and issuances | 0.1 | 2.4
Settlements and sales | (1.6) | (0.1)
Increase (decrease) in fair value included in net income | 0.1 | 0.0
Balance at the end of the year |  | 5.0
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 |
Balance at the end of the year |  | $ 0.0
Transfers in/(out) | $ (19.5) |

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