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Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | $ 8,620.0 | $ 7,985.3
Equity securities | 150.1 | 130.9
Other investments | 760.6 | 796.9
U.S. Treasury and Government Agencies [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 576.8 | 462.6
Municipals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,049.3 | 1,185.2
Corporates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 3,973.9 | 3,849.7
Residential Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,728.8 | 1,310.9
Commercial Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 586.2 | 818.2
Other Asset-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 703.2 | 356.5
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 8,620.0 | 7,985.3
Equity securities | 150.1 | 130.9
Other investments | 3.8 | 3.8
Total investment assets at fair value | 8,773.9 | 8,120.0
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 | 576.8 | 462.6
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1.8 | 2.2
Fair Value, Measurements, Recurring [Member] | Municipals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,049.3 | 1,185.2
Fair Value, Measurements, Recurring [Member] | Corporates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 3,973.9 | 3,849.7
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 | 1,478.4 | 1,168.7
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 | 250.4 | 142.2
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 586.2 | 818.2
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 703.2 | 356.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 436.2 | 324.3
Equity securities | 142.3 | 123.3
Total investment assets at fair value | 578.5 | 447.6
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 | 436.2 | 324.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 8,169.6 | 7,645.3
Total investment assets at fair value | 8,169.6 | 7,645.3
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 | 140.6 | 138.3
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 | 1.8 | 2.2
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 | 1,041.3 | 1,176.6
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 | 3,973.9 | 3,849.6
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 | 1,478.4 | 1,168.7
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 | 250.4 | 142.2
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 | 580.0 | 811.2
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 | 703.2 | 356.5
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities | 14.2 | 15.7
Equity securities | 7.8 | 7.6
Other investments | 3.8 | 3.8
Total investment assets at fair value | 25.8 | 27.1
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 | 8.0 | 8.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporates [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fixed maturities |  | 0.1
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 | $ 6.2 | $ 7.0

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