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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value | $ 24.2 | $ 14.9 |
Catastrophe Bonds, Fair Value Disclosure | 1.0 | 1.6 |
Derivative assets | 17.0 | 31.7 |
Fair Market Value | 4,978.3 | 4,231.1 |
Fair Market Value | 1,488.7 | 1,964.4 |
Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 111.7 | 117.1 |
Privately-held Investments, at fair value | 286.8 |  |
Total | 6,701.7 | 6,410.7 |
Recurring | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivative assets | 17.0 | 31.7 |
Recurring | Liabilities under derivative contracts | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | (45.9) | (9.3) |
Recurring | Liabilities under derivative contracts | Loss Portfolio Transfer |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | (3.6) | (16.5) |
Recurring | Fixed income securities | Non-agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 4.4 |  |
Recurring | Fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 4,692.2 | 4,122.6 |
Recurring | Short-term investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 261.9 | 93.6 |
Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,199.9 | 1,485.7 |
Catastrophe Bonds, Fair Value Disclosure | 1.0 | 1.6 |
Recurring | Short-term investments trading at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1.0 |  |
Recurring | Real estate fund |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 267.2 | 209.3 |
Recurring | Privately-held investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value | 24.2 |  |
Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | 0.0 |  |
Total | 2,207.8 | 1,748.4 |
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivative assets | 0.0 | 0.0 |
Recurring | Level 1 | Liabilities under derivative contracts | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | 0.0 | 0.0 |
Recurring | Level 1 | Liabilities under derivative contracts | Loss Portfolio Transfer |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | 0.0 | 0.0 |
Recurring | Level 1 | Fixed income securities | Non-agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Recurring | Level 1 | Fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,675.7 | 1,403.0 |
Recurring | Level 1 | Short-term investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 260.2 | 86.7 |
Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 270.9 | 258.5 |
Catastrophe Bonds, Fair Value Disclosure | 0.0 | 0.0 |
Recurring | Level 1 | Short-term investments trading at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1.0 |  |
Recurring | Level 1 | Real estate fund |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Recurring | Level 1 | Privately-held investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value | 0.0 |  |
Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | 0.0 |  |
Total | 3,919.3 | 3,979.6 |
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivative assets | 17.0 | 31.7 |
Recurring | Level 2 | Liabilities under derivative contracts | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | (45.9) | (9.3) |
Recurring | Level 2 | Liabilities under derivative contracts | Loss Portfolio Transfer |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | 0.0 | 0.0 |
Recurring | Level 2 | Fixed income securities | Non-agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 4.4 |  |
Recurring | Level 2 | Fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 3,016.5 | 2,719.6 |
Recurring | Level 2 | Short-term investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1.7 | 6.9 |
Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 929.0 | 1,227.2 |
Catastrophe Bonds, Fair Value Disclosure | 1.0 | 1.6 |
Recurring | Level 2 | Short-term investments trading at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Recurring | Level 2 | Real estate fund |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Recurring | Level 2 | Privately-held investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value | 0.0 |  |
Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, at fair value | 286.8 |  |
Total | 307.4 | 473.4 |
Change in unrealized investment gains (losses) | 1.3 | (15.7) |
Fair Market Value | 311.0 | 489.9 |
Recurring | Level 3 | Liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities at beginning of year | (3.6) | (16.5) | $ (31.7)
Liability purchases | 0.0 | 0.0 |
Liability transfers | 0.0 | 0.0 |
Liability, Transfers out of Level 3 | 0.0 | 0.0 |
Liability sales | 0.0 | 0.0 |
Liability, increase (decrease) in fv included in net income | 12.9 | 15.2 |
Change in unrealized gains or losses | 12.9 | 15.2 |
Recurring | Level 3 | Loss Portfolio Transfer | Liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities at beginning of year | (3.6) | (16.5) | (31.7)
Liability purchases | 0.0 | 0.0 |
Liability transfers | 0.0 | 0.0 |
Liability, Transfers out of Level 3 | 0.0 | 0.0 |
Liability sales | 0.0 | 0.0 |
Liability, increase (decrease) in fv included in net income | 12.9 | 15.2 |
Change in unrealized gains or losses | 12.9 | 15.2 |
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivative assets | 0.0 | 0.0 |
Recurring | Level 3 | Liabilities under derivative contracts | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | 0.0 | 0.0 |
Recurring | Level 3 | Liabilities under derivative contracts | Loss Portfolio Transfer |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Liabilities under derivative contracts, fair value | (3.6) | (16.5) |
Recurring | Level 3 | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 71.4 | 113.7 |
Transfers Into Level 3 | 0.0 | 5.3 |
Transfers out of Level 3 | 0.0 | 12.1 |
Settlements and sales | (239.7) | (137.2) |
Increase (decrease) in fair value included in net income | (10.6) | (12.8) |
Fair Market Value |  | 489.9 | 533.0
Recurring | Level 3 | Fixed income securities | Non-agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Recurring | Level 3 | Fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Recurring | Level 3 | Short-term investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Catastrophe Bonds, Fair Value Disclosure | 0.0 | 0.0 |
Recurring | Level 3 | Short-term investments trading at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Recurring | Level 3 | Real estate fund |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Recurring | Level 3 | Commercial mortgage loans |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 0.0 | (17.9) |
Fair Market Value | 79.7 | 274.9 |
Recurring | Level 3 | Commercial mortgage loans | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 0.5 | 40.6 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (184.4) | (61.5) |
Increase (decrease) in fair value included in net income | (11.3) | (16.3) |
Fair Market Value |  | 274.9 | 312.1
Recurring | Level 3 | Middle market loans and other private debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 0.0 | 0.5 |
Fair Market Value | 61.0 | 84.8 |
Recurring | Level 3 | Middle market loans and other private debt | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 0.4 | 18.3 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (35.5) | (41.9) |
Increase (decrease) in fair value included in net income | 0.4 | 1.5 |
Fair Market Value |  | 84.8 | 106.9
Recurring | Level 3 | Global corporate securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 0.0 | (0.2) |
Fair Market Value | 18.5 | 14.4 |
Recurring | Level 3 | Global corporate securities | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 4.4 | 0.0 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (0.3) | (0.4) |
Increase (decrease) in fair value included in net income | 0.0 | (0.2) |
Fair Market Value |  | 14.4 | 15.0
Recurring | Level 3 | Equity Securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) |  | 0.0 |
Fair Market Value |  | 0.0 |
Recurring | Level 3 | Equity Securities | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances |  | 0.0 |
Transfers Into Level 3 |  | 0.0 |
Transfers out of Level 3 |  | (6.6) |
Settlements and sales |  | 0.0 |
Increase (decrease) in fair value included in net income |  | 0.0 |
Fair Market Value |  |  | 6.6
Recurring | Level 3 | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 0.0 | 0.0 |
Fair Market Value | 0.0 | 18.0 |
Recurring | Level 3 | Short-term investments | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 0.0 | 18.2 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (7.1) | (25.8) |
Increase (decrease) in fair value included in net income | 0.0 | 0.0 |
Fair Market Value |  | 18.0 | 25.6
Recurring | Level 3 | Asset-backed |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 1.1 | 1.7 |
Fair Market Value | 127.6 | 82.9 |
Recurring | Level 3 | Asset-backed | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 56.1 | 21.9 |
Transfers Into Level 3 | 0.0 | 5.3 |
Transfers out of Level 3 | 0.0 | (5.5) |
Settlements and sales | (12.4) | (7.6) |
Increase (decrease) in fair value included in net income | 1.0 | 2.0 |
Fair Market Value |  | 82.9 | 66.8
Recurring | Level 3 | Privately-held investments — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value | 24.2 |  |
U.S. government |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 1,480.6 | 1,202.6 |
Fair Market Value | 261.3 | 245.5 |
U.S. government | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,480.6 | 1,202.6 |
U.S. government | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 261.3 | 245.5 |
U.S. government | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,480.6 | 1,202.6 |
U.S. government | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 261.3 | 245.5 |
U.S. government | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
U.S. government | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
U.S. government | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
U.S. government | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
U.S. agency |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 7.2 | 7.2 |
U.S. agency | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 7.2 | 7.2 |
U.S. agency | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
U.S. agency | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 7.2 | 7.2 |
U.S. agency | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Municipal |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 82.3 | 128.1 |
Fair Market Value | 1.6 | 3.1 |
Municipal | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 82.3 | 128.1 |
Municipal | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1.6 | 3.1 |
Municipal | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Municipal | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Municipal | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 82.3 | 128.1 |
Municipal | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1.6 | 3.1 |
Municipal | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Municipal | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Corporate |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 1,986.4 | 1,959.3 |
Fair Market Value | 151.1 | 171.5 |
Corporate | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,986.4 | 1,959.3 |
Corporate | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 151.1 | 171.5 |
Corporate | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Corporate | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Corporate | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,986.4 | 1,959.3 |
Corporate | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 151.1 | 171.5 |
Corporate | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Corporate | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-U.S. government-backed corporate |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 131.3 | 100.7 |
Fair Market Value | 2.8 | 8.3 |
Non-U.S. government-backed corporate | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 131.3 | 100.7 |
Non-U.S. government-backed corporate | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 2.8 | 8.3 |
Non-U.S. government-backed corporate | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-U.S. government-backed corporate | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-U.S. government-backed corporate | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 131.3 | 100.7 |
Non-U.S. government-backed corporate | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 2.8 | 8.3 |
Non-U.S. government-backed corporate | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-U.S. government-backed corporate | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-U.S. government |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 246.8 | 273.8 |
Fair Market Value | 24.4 | 34.8 |
Non-U.S. government | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 246.8 | 273.8 |
Non-U.S. government | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 24.4 | 34.8 |
Non-U.S. government | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 195.1 | 200.4 |
Non-U.S. government | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 9.6 | 13.0 |
Non-U.S. government | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 51.7 | 73.4 |
Non-U.S. government | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 14.8 | 21.8 |
Non-U.S. government | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-U.S. government | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Asset-backed |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 234.5 |  |
Fair Market Value | 625.2 | 908.2 |
Asset-backed | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 234.5 |  |
Asset-backed | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 625.2 | 908.2 |
Asset-backed | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Asset-backed | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Asset-backed | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 234.5 |  |
Asset-backed | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 625.2 | 908.2 |
Asset-backed | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Asset-backed | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Non-agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 4.4 | 5.8 |
Non-agency commercial mortgage-backed securities | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 5.8 |
Non-agency commercial mortgage-backed securities | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Non-agency commercial mortgage-backed securities | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 5.8 |
Non-agency commercial mortgage-backed securities | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Agency residential mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 518.7 | 445.1 |
Agency residential mortgage-backed securities | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 518.7 |  |
Agency residential mortgage-backed securities | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Agency residential mortgage-backed securities | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 518.7 |  |
Agency residential mortgage-backed securities | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Agency mortgage-backed |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 31.1 | 22.2 |
Agency mortgage-backed | Recurring | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 445.1 |
Agency mortgage-backed | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 31.1 | 22.2 |
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Agency mortgage-backed | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Agency mortgage-backed | Recurring | Level 2 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 445.1 |
Agency mortgage-backed | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 31.1 | 22.2 |
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Agency mortgage-backed | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
High yield loans |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 102.4 | 92.1 |
High yield loans | Recurring | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 102.4 | 92.1 |
High yield loans | Recurring | Level 1 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
High yield loans | Recurring | Level 2 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 102.4 | 92.1 |
High yield loans | Recurring | Level 3 | Fixed income securities, trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Fixed income securities, trading | Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value |  | 14.9 |
Privately-held Investments, at fair value |  | 475.0 |
Fixed income securities, trading | Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value |  | 0.0 |
Privately-held Investments, at fair value |  | 0.0 |
Fixed income securities, trading | Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value |  | 0.0 |
Privately-held Investments, at fair value |  | 0.0 |
Fixed income securities, trading | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value |  | 14.9 |
Privately-held Investments, at fair value |  | 475.0 |
Short-term investments trading at fair value | Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 2.1 |
Short-term investments trading at fair value | Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.2 |
Short-term investments trading at fair value | Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 1.9 |
Short-term investments trading at fair value | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Asset backed securities, privately-held afs | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value | 24.2 | 14.9 |
Change in unrealized investment gains (losses) | 0.2 | 0.2 |
Asset backed securities, privately-held afs | Recurring | Level 3 | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Market Value |  | 14.9 | $ 0.0
Purchases and issuances | 10.0 | 14.7 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | 0.0 | 0.0 |
Increase (decrease) in fair value included in net income | $ (0.7) | $ 0.2 |

X
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