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Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | $ 9,536.5 | $ 8,542.2
Equity securities | 184.8 | 157.7
Other investments | 661.4 | 709.9
U.S. Treasury and Government Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 455.5 | 494.2
Municipals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 847.7 | 893.8
Corporates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,253.5 | 3,805.9
Residential Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 2,421.8 | 2,124.1
Commercial Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 589.9 | 522.2
Other asset-backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 965.4 | 700.1
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 9,536.5 | 8,542.2
Equity securities | 184.8 | 157.7
Other investments | 16.8 | 3.9
Total investment assets at fair value | 9,738.1 | 8,703.8
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Government Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 455.5 | 494.2
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 2.7 | 1.9
Fair Value, Measurements, Recurring [Member] | Municipals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 847.7 | 893.8
Fair Value, Measurements, Recurring [Member] | Corporates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,253.5 | 3,805.9
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed, U.S. Agency Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,903.2 | 1,790.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed, Non-Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 518.6 | 333.4
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 589.9 | 522.2
Fair Value, Measurements, Recurring [Member] | Other asset-backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 965.4 | 700.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 331.1 | 363.9
Equity securities | 164.3 | 136.9
Total investment assets at fair value | 495.4 | 500.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury and Government Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 331.1 | 363.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 9,193.1 | 8,164.8
Other investments | 13.4 |
Total investment assets at fair value | 9,206.5 | 8,164.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury and Government Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 124.4 | 130.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign governments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 2.7 | 1.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 840.3 | 886.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,253.5 | 3,805.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed, U.S. Agency Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,903.2 | 1,790.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed, Non-Agency [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 518.6 | 333.4
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 585.0 | 516.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Other asset-backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 965.4 | 700.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 12.3 | 13.5
Equity securities | 20.5 | 20.8
Other investments | 3.4 | 3.9
Total investment assets at fair value | 36.2 | 38.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 7.4 | 7.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | $ 4.9 | $ 5.9

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