Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,985.3	$ 7,481.8
Equity securities	130.9	241.9
Other investments	796.9	786.1
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	462.6	420.2
Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.2	2.2
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,185.2	1,076.1
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,849.7	3,729.9
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,310.9	1,073.8
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	818.2	836.4
Other asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	356.5	343.2
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,985.3	7,481.8
Equity securities	130.9	241.9
Other investments	3.8	3.6
Total investment assets at fair value	8,120.0	7,727.3
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	462.6	420.2
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.2	2.2
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,185.2	1,076.1
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,849.7	3,729.9
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	1,168.7	974.0
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	142.2	99.8
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	818.2	836.4
Fair Value, Measurements, Recurring [Member] | Other asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	356.5	343.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	324.3	282.7
Equity securities	123.3	234.4
Total investment assets at fair value	447.6	517.1
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	324.3	282.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,645.3	7,182.2
Total investment assets at fair value	7,645.3	7,182.2
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	138.3	137.5
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.2	2.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,176.6	1,066.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,849.6	3,729.8
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	1,168.7	974.0
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	142.2	99.8
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	811.2	828.9
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	356.5	343.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	15.7	16.9
Equity securities	7.6	7.5
Other investments	3.8	3.6
Total investment assets at fair value	27.1	28.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	8.6	9.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.1	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	$ 7.0	$ 7.5