Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 9,835.7	$ 9,536.5
Equity securities	197.1	184.8
Other investments	638.6	661.4
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	492.9	455.5
Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	988.9	847.7
Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,416.7	4,253.5
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2,155.3	2,421.8
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	617.0	589.9
Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,149.3	965.4
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	9,835.7	9,536.5
Equity securities	197.1	184.8
Other investments	16.9	16.8
Total investment assets at fair value	10,049.7	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	492.9	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	15.6	2.7
Fair Value, Measurements, Recurring [Member] \| Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	988.9	847.7
Fair Value, Measurements, Recurring [Member] \| Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,416.7	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	1,724.4	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	430.9	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	617.0	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	1,149.3	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	384.8	331.1
Equity securities	183.4	164.3
Total investment assets at fair value	568.2	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	384.8	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	9,440.0	9,193.1
Other investments	13.5	13.4
Total investment assets at fair value	9,453.5	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	108.1	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	15.6	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	982.5	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	4,416.7	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	1,724.4	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	430.9	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	612.5	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	1,149.3	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	10.9	12.3
Equity securities	13.7	20.5
Other investments	3.4	3.4
Total investment assets at fair value	28.0	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	6.4	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	$ 4.5	$ 4.9