Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 10,061.1	$ 9,536.5
Equity securities	203.3	184.8
Other investments	637.6	661.4
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	434.9	455.5
Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,003.1	847.7
Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,515.2	4,253.5
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2,236.0	2,421.8
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	802.0	589.9
Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,052.6	965.4
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	10,061.1	9,536.5
Equity securities	203.3	184.8
Other investments	16.9	16.8
Total investment assets at fair value	10,281.3	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	434.9	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	17.3	2.7
Fair Value, Measurements, Recurring [Member] \| Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,003.1	847.7
Fair Value, Measurements, Recurring [Member] \| Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,515.2	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	1,771.6	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	464.4	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	802.0	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	1,052.6	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	321.2	331.1
Equity securities	183.0	164.3
Total investment assets at fair value	504.2	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	321.2	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	9,728.5	9,193.1
Other investments	13.5	13.4
Total investment assets at fair value	9,742.0	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	113.7	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	17.3	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	996.4	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	4,515.2	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	1,771.6	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	464.4	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	797.3	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	1,052.6	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	11.4	12.3
Equity securities	20.3	20.5
Other investments	3.4	3.4
Total investment assets at fair value	35.1	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	6.7	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	$ 4.7	$ 4.9