v3.26.1
Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities $ 9,976.2 $ 9,536.5
Equity securities 188.2 184.8
Other investments 637.3 661.4
U.S. Treasury and Government Agencies [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 438.7 455.5
Municipals [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 893.2 847.7
Corporates [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 4,348.1 4,253.5
Residential Mortgage-Backed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 2,324.3 2,421.8
Commercial Mortgage-Backed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 854.9 589.9
Other Asset-Backed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 1,114.1 965.4
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 9,976.2 9,536.5
Equity securities 188.2 184.8
Other investments 16.9 16.8
Total investment assets at fair value 10,181.3 9,738.1
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 438.7 455.5
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 2.9 2.7
Fair Value, Measurements, Recurring [Member] | Municipals [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 893.2 847.7
Fair Value, Measurements, Recurring [Member] | Corporates [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 4,348.1 4,253.5
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 1,841.7 1,903.2
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 482.6 518.6
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 854.9 589.9
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 1,114.1 965.4
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 324.4 331.1
Equity securities 167.6 164.3
Total investment assets at fair value 492.0 495.4
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 324.4 331.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 9,639.9 9,193.1
Other investments 13.5 13.4
Total investment assets at fair value 9,653.4 9,206.5
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 114.3 124.4
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 2.9 2.7
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 886.1 840.3
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 4,348.1 4,253.5
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 1,841.7 1,903.2
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 482.6 518.6
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 850.1 585.0
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 1,114.1 965.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturities 11.9 12.3
Equity securities 20.6 20.5
Other investments 3.4 3.4
Total investment assets at fair value 35.9 36.2
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 7.1 7.4
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 $ 4.8 $ 4.9