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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments, available for sale, at fair value | $ 14.9 | $ 0.0 |
Catastrophe Bonds, Fair Value Disclosure | 1.6 | 5.1 |
Derivatives at fair value | 31.7 | 56.2 |
Fair market value | 4,122.6 | 3,788.6 |
Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 2.1 | 6.3 |
Commercial mortgage loans |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 274.9 | 312.1 |
Middle market loans [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 84.8 | 106.9 |
Equity securities — Trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 6.6 |
Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 18.0 | 25.6 |
Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 117.1 | 134.6 |
Privately-held Investments trading, at fair value | 475.0 |  |
Total | 6,410.7 | 6,100.9 |
Recurring | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivatives at fair value | 31.7 | 56.2 |
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 | (9.3) | (3.2) |
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 | (16.5) | (31.7) |
Recurring | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 5.8 |  |
Recurring | Fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 4,122.6 | 3,788.6 |
Recurring | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 93.6 | 52.0 |
Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,485.7 | 1,475.5 |
Catastrophe Bonds, Fair Value Disclosure | 1.6 | 2.9 |
Recurring | Short-term investments trading at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 2.1 |  |
Recurring | Real estate fund |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 209.3 | 221.3 |
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 | 14.9 |  |
Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments trading, at fair value | 0.0 |  |
Total | 1,748.4 | 1,428.7 |
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivatives at fair value |  | 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 | Total fixed income securities — Available for sale | 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 | 1,403.0 | 1,098.3 |
Recurring | Level 1 | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 86.7 | 51.9 |
Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 258.5 | 272.2 |
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 | 0.2 |  |
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 trading, at fair value | 0.0 |  |
Total | 3,979.6 | 3,949.6 |
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivatives at fair value | 31.7 | 56.2 |
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 | (9.3) | (3.2) |
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 |
Recurring | Level 2 | Total fixed income securities — Available for sale | Agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 5.8 |  |
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 | 2,719.6 | 2,690.3 |
Recurring | Level 2 | Short-term investments |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 6.9 | 0.1 |
Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,227.2 | 1,203.3 |
Catastrophe Bonds, Fair Value Disclosure | 1.6 | 2.9 |
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 | 1.9 |  |
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 trading, at fair value | 475.0 |  |
Total | 473.4 | 501.3 |
Change in unrealized investment gains (losses) | (15.7) | (4.6) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 489.9 | 533.0 |
Recurring | Level 3 | Liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value | (16.5) | (31.7) | $ 0.0
Liability purchases | 0.0 | (17.2) |
Liability transfers | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, 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 | 15.2 | (14.5) |
Change in unrealized gains or losses | 15.2 | 0.0 |
Recurring | Level 3 | Loss Portfolio Transfer | Liability |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value | (16.5) | (31.7) | 0.0
Liability purchases | 0.0 | (17.2) |
Liability transfers | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, 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 | 15.2 | (14.5) |
Change in unrealized gains or losses | 15.2 | 0.0 |
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Derivatives at fair value |  | 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 | (16.5) | (31.7) |
Recurring | Level 3 | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 113.7 | 378.2 |
Transfers in/(out) | 5.3 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | (12.1) | 0.0 |
Settlements and sales | (137.2) | (147.4) |
Increase (decrease) in fair value included in net income | (12.8) | (4.9) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 533.0 | 307.1
Recurring | Level 3 | Total fixed income securities — Available for sale | 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 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
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) | (17.9) | (0.5) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 274.9 | 312.1 |
Recurring | Level 3 | Commercial mortgage loans | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 40.6 | 215.7 |
Transfers in/(out) | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (61.5) | (113.1) |
Increase (decrease) in fair value included in net income | (16.3) | (2.0) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 312.1 | 211.5
Recurring | Level 3 | Middle market loans [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 0.5 | (2.1) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 84.8 | 106.9 |
Recurring | Level 3 | Middle market loans [Member] | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 18.3 | 61.8 |
Transfers in/(out) | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (41.9) | (19.3) |
Increase (decrease) in fair value included in net income | 1.5 | (0.9) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 106.9 | 65.3
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) | (0.2) | 0.0 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 14.4 | 15.0 |
Recurring | Level 3 | Global corporate securities | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 0.0 | 15.1 |
Transfers in/(out) | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (0.4) | 0.0 |
Increase (decrease) in fair value included in net income | (0.2) | (0.1) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 15.0 | 0.0
Recurring | Level 3 | Equity securities — Trading |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 0.0 | 0.0 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 0.0 | 6.6 |
Recurring | Level 3 | Equity securities — Trading | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 0.0 | 5.5 |
Transfers in/(out) | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | (6.6) | 0.0 |
Settlements and sales | 0.0 | (2.4) |
Increase (decrease) in fair value included in net income | 0.0 | (0.1) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 6.6 | 3.6
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 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 18.0 | 25.6 |
Recurring | Level 3 | Short-term investments | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 18.2 | 25.6 |
Transfers in/(out) | 0.0 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 0.0 | 0.0 |
Settlements and sales | (25.8) | 0.0 |
Increase (decrease) in fair value included in net income | 0.0 | 0.0 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 25.6 | 0.0
Recurring | Level 3 | Asset-backed |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Change in unrealized investment gains (losses) | 1.7 | (2.0) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 82.9 | 66.8 |
Recurring | Level 3 | Asset-backed | Assets |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Purchases and issuances | 21.9 | 54.5 |
Transfers in/(out) | 5.3 | 0.0 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | (5.5) | 0.0 |
Settlements and sales | (7.6) | (12.6) |
Increase (decrease) in fair value included in net income | 2.0 | (1.8) |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) |  | 66.8 | $ 26.7
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 | 14.9 |  |
U.S. government |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair market value | 1,202.6 | 953.0 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 245.5 | 261.6 |
U.S. government | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,202.6 | 953.0 |
U.S. government | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 245.5 | 261.6 |
U.S. government | Recurring | Level 1 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,202.6 | 953.0 |
U.S. government | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 245.5 | 261.6 |
U.S. government | Recurring | Level 2 | Total 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 |
U.S. government | Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
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 | Total 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 |
U.S. government | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
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 | 8.8 |
U.S. agency | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 7.2 | 8.8 |
U.S. agency | Recurring | Level 1 | Total 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 |
U.S. agency | Recurring | Level 2 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 7.2 | 8.8 |
U.S. agency | Recurring | Level 3 | Total 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 |
Municipal |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair market value | 128.1 | 149.5 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 3.1 | 3.6 |
Municipal | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 128.1 | 149.5 |
Municipal | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 3.1 | 3.6 |
Municipal | Recurring | Level 1 | Total 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 |
Municipal | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Municipal | Recurring | Level 2 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 128.1 | 149.5 |
Municipal | Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 3.1 | 3.6 |
Municipal | Recurring | Level 3 | Total 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 |
Municipal | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
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 | 1,959.3 | 1,845.0 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 171.5 | 162.1 |
Corporate | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,959.3 | 1,845.0 |
Corporate | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 171.5 | 162.1 |
Corporate | Recurring | Level 1 | Total 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 |
Corporate | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Corporate | Recurring | Level 2 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 1,959.3 | 1,845.0 |
Corporate | Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 171.5 | 162.1 |
Corporate | Recurring | Level 3 | Total 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 |
Corporate | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
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 | 100.7 | 110.4 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 8.3 | 11.6 |
Non-U.S. government-backed corporate | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 100.7 | 110.4 |
Non-U.S. government-backed corporate | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 8.3 | 11.6 |
Non-U.S. government-backed corporate | Recurring | Level 1 | Total 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 |
Non-U.S. government-backed corporate | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
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 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 100.7 | 110.4 |
Non-U.S. government-backed corporate | Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 8.3 | 11.6 |
Non-U.S. government-backed corporate | Recurring | Level 3 | Total 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 |
Non-U.S. government-backed corporate | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
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 | 273.8 | 213.6 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 34.8 | 30.4 |
Non-U.S government | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 273.8 | 213.6 |
Non-U.S government | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 34.8 | 30.4 |
Non-U.S government | Recurring | Level 1 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 200.4 | 145.3 |
Non-U.S government | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 13.0 | 10.6 |
Non-U.S government | Recurring | Level 2 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 73.4 | 68.3 |
Non-U.S government | Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 21.8 | 19.8 |
Non-U.S government | Recurring | Level 3 | Total 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 |
Non-U.S government | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
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] |  |  |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 908.2 | 896.5 |
Asset-backed | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 908.2 | 896.5 |
Asset-backed | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
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 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 908.2 | 896.5 |
Asset-backed | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Agency commercial mortgage-backed securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair market value | 5.8 | 5.6 |
Agency commercial mortgage-backed securities | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 5.6 |
Agency commercial mortgage-backed securities | Recurring | Level 1 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Agency commercial mortgage-backed securities | Recurring | Level 2 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 5.6 |
Agency commercial mortgage-backed securities | Recurring | Level 3 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Residential Mortgage-Backed Securities |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair market value | 445.1 |  |
Residential Mortgage-Backed Securities | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 445.1 |  |
Residential Mortgage-Backed Securities | Recurring | Level 1 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 |  |
Residential Mortgage-Backed Securities | Recurring | Level 2 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 445.1 |  |
Residential Mortgage-Backed Securities | Recurring | Level 3 | Total fixed income securities — Available for sale |  |  |
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 | 445.1 | 502.7 |
Fixed income maturities, trading at fair value (amortized cost — $1,106.6 and $1,205.0) | 22.2 | 21.4 |
Agency mortgage-backed | Recurring | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 502.7 |
Agency mortgage-backed | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 22.2 | 21.4 |
Agency mortgage-backed | Recurring | Level 1 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Agency mortgage-backed | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
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 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 502.7 |
Agency mortgage-backed | Recurring | Level 2 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 22.2 | 21.4 |
Agency mortgage-backed | Recurring | Level 3 | Total fixed income securities — Available for sale |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 0.0 |
Agency mortgage-backed | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
High yield loans | Recurring | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 92.1 | 88.3 |
High yield loans | Recurring | Level 1 | Held for trading financial assets, at fair value |  |  |
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 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 92.1 | 88.3 |
High yield loans | Recurring | Level 3 | Held for trading financial assets, at fair value |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value | 0.0 | 0.0 |
Held for trading financial assets, at fair value | Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments trading, at fair value |  | 533.0 |
Held for trading financial assets, at fair value | Recurring | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments trading, at fair value |  | 0.0 |
Held for trading financial assets, at fair value | Recurring | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments trading, at fair value |  | 0.0 |
Held for trading financial assets, at fair value | Recurring | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Privately-held Investments trading, at fair value |  | 533.0 |
Short-term investments trading at fair value | Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments, fair value |  | 6.3 |
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 |  | 6.3 |
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 |  | 0.0 |
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 | 14.9 |  |
Change in unrealized investment gains (losses) | 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 |  | $ 0.0 |
Purchases and issuances | 14.7 |  |
Transfers in/(out) | 0.0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 | 0.0 |  |
Settlements and sales | 0.0 |  |
Increase (decrease) in fair value included in net income | $ 0.2 |  |

X
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