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Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
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