Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	$ 192.3	$ 230.0
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	$ 2.8	(1.5)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net Realized Investment Gains (Losses)
Included in Other Comprehensive Income	$ 0.2	6.8
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Changes in Unrealized (Losses) Gains on Investment Securities with:
Purchases	$ 133.4	51.6
Sales	(105.7)	(102.6)
Transfers into Level 3	10.6	8.1
Transfers out of Level 3	(24.6)	(0.1)
Balance, End of Year	209.0	192.3
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	177.1	216.0
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	0.6	(0.7)
Included in Other Comprehensive Income	0.8	6.4
Purchases	124.2	50.4
Sales	(104.0)	(102.6)
Transfers into Level 3	7.1	7.7
Transfers out of Level 3	(11.6)	(0.1)
Balance, End of Year	194.2	177.1
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	0.1	0.0
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	0.0	0.0
Included in Other Comprehensive Income	(0.5)	0.0
Purchases	0.0	0.1
Sales	0.0	0.0
Transfers into Level 3	3.5	0.0
Transfers out of Level 3	(1.3)	0.0
Balance, End of Year	1.8	0.1
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	7.1	6.8
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	0.0	0.0
Included in Other Comprehensive Income	0.1	0.3
Purchases	1.9	0.0
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	(4.9)	0.0
Balance, End of Year	4.2	7.1
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	0.0
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	0.0
Included in Other Comprehensive Income	0.0
Purchases	6.8
Sales	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	(6.8)
Balance, End of Year	0.0	0.0
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	5.2	5.1
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	0.0	0.0
Included in Other Comprehensive Income	(0.2)	0.1
Purchases	0.0	0.0
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Balance, End of Year	5.0	5.2
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Year	2.8	2.1
Total Gains (Losses):
Included in Consolidated Statements of Income (Loss)	2.2	(0.8)
Included in Other Comprehensive Income	0.0	0.0
Purchases	0.5	1.1
Sales	(1.7)	0.0
Transfers into Level 3	0.0	0.4
Transfers out of Level 3	0.0	0.0
Balance, End of Year	$ 3.8	$ 2.8