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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, 2023 | Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Beginning of the year | $ 3,788.6 |
End of the year | $ 4,122.6 | $ 3,788.6
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 |
Balance at the end of the year | 274.9 | 312.1
Middle market loans |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 106.9 |
Balance at the end of the year | 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 |
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 | 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) | (15.7) | (4.6)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 533.0 |
Balance at the end of the year | 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 | (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 | 15.2 | (14.5)
Balance at the end of the period | (16.5) | (31.7)
Change in unrealized gains or losses | 15.2 | 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 | 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 | 15.2 | (14.5)
Balance at the end of the period | (16.5) | (31.7)
Change in unrealized gains or losses | 15.2 | 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3 | 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) | (17.9) | (0.5)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 312.1 |
Balance at the end of the year | 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) | 0.5 | (2.1)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 106.9 |
Balance at the end of the year | 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) | 1.7 | (2.0)
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 66.8 |
Balance at the end of the year | 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) | (0.2) | 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 | 14.4 | 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) | 0.0 | 0.0
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |
Balance at the beginning of the year | 6.6 |
Balance at the end of the year | 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
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 | 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 | 533.0 | 307.1
Purchases and issuances | 113.7 | 378.2
Transfers in/(out) | 5.3 | 0.0
Settlements and sales | (137.2) | (147.4)
Increase (decrease) in fair value included in net income | (12.8) | (4.9)
Balance at the end of the year |  | 533.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | (12.1) | 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 | 312.1 | 211.5
Purchases and issuances | 40.6 | 215.7
Transfers in/(out) | 0.0 | 0.0
Settlements and sales | (61.5) | (113.1)
Increase (decrease) in fair value included in net income | (16.3) | (2.0)
Balance at the end of the year |  | 312.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 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
Purchases and issuances | 18.3 | 61.8
Transfers in/(out) | 0.0 | 0.0
Settlements and sales | (41.9) | (19.3)
Increase (decrease) in fair value included in net income | 1.5 | (0.9)
Balance at the end of the year |  | 106.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 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
Purchases and issuances | 21.9 | 54.5
Transfers in/(out) | 5.3 | 0.0
Settlements and sales | (7.6) | (12.6)
Increase (decrease) in fair value included in net income | 2.0 | (1.8)
Balance at the end of the year |  | 66.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | (5.5) | 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
Transfers in/(out) | 0.0 | 0.0
Settlements and sales | (0.4) | 0.0
Increase (decrease) in fair value included in net income | (0.2) | (0.1)
Balance at the end of the year |  | 15.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 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
Purchases and issuances | 0.0 | 5.5
Transfers in/(out) | 0.0 | 0.0
Settlements and sales | 0.0 | (2.4)
Increase (decrease) in fair value included in net income | 0.0 | (0.1)
Balance at the end of the year |  | 6.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | (6.6) | 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 | 25.6 | 0.0
Purchases and issuances | 18.2 | 25.6
Transfers in/(out) | 0.0 | 0.0
Settlements and sales | (25.8) | 0.0
Increase (decrease) in fair value included in net income | 0.0 | 0.0
Balance at the end of the year |  | 25.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | $ 0.0 | $ 0.0

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- Definition Indicates line item in statement in which net income is reported that includes unrealized gain (loss) from asset measured at fair value using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of unrealized gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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