Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,678.2	$ 7,481.8
Equity securities	234.8	241.9
Other investments	795.0	786.1
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	432.9	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	1,125.8	1,076.1
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,779.9	3,729.9
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,153.1	1,073.8
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	842.5	836.4
Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	341.8	343.2
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,678.2	7,481.8
Equity securities	234.8	241.9
Other investments	3.6	3.6
Total investment assets at fair value	7,916.6	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	432.9	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	1,125.8	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	3,779.9	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	1,022.1	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	131.0	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	842.5	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	341.8	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	295.8	282.7
Equity securities	227.3	234.4
Total investment assets at fair value	523.1	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	295.8	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	7,365.7	7,182.2
Total investment assets at fair value	7,365.7	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	137.1	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	1,116.7	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	3,779.8	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	1,022.1	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	131.0	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	835.0	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	341.8	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	16.7	16.9
Equity securities	7.5	7.5
Other investments	3.6	3.6
Total investment assets at fair value	27.8	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	9.1	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	$ 7.5	$ 7.5