Fair Value of Financial Instruments - Transfers Between Different Fair Value Levels (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net investment losses	Net investment losses
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Change in net unrealized investment gains (losses) on fixed maturity securities	Change in net unrealized investment gains (losses) on fixed maturity securities
Financial Assets
Beginning balance	$ 518.5	$ 425.3
Transfers into Level 3	48.7	79.4
Transfers out of Level 3	(12.4)	(3.7)
Total gains or losses
Net investment gains (losses) included in net income	0.9	0.4
Net unrealized investment gains (losses) included in OCI	3.7	(0.8)
Purchases	88.3	76.0
Issuances	0.0	0.0
Sales	(46.4)	(7.7)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(68.3)	(50.4)
Ending balance	533.0	518.5
Financial Liabilities
Beginning balance	82.4	91.3
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	$ 5.4	$ 1.3
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Change in net unrealized investment gains (losses) on fixed maturity securities	Change in net unrealized investment gains (losses) on fixed maturity securities
Net unrealized investment gains (losses) included in OCI	$ 0.0	$ 0.4
Purchases	0.0	0.0
Issuances	5.4	8.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(17.7)	(18.6)
Ending balance	75.5	82.4
Municipal Bonds
Financial Assets
Beginning balance	74.0	54.4
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(2.1)	11.3
Purchases	4.2	9.5
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(1.2)	(1.2)
Ending balance	74.9	74.0
Corporate  Bonds
Financial Assets
Beginning balance	342.5	261.3
Transfers into Level 3	11.8	76.5
Transfers out of Level 3	(8.4)	(3.7)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0
Net unrealized investment gains (losses) included in OCI	5.7	(17.1)
Purchases	81.2	64.5
Issuances	0.0	0.0
Sales	(46.4)	(7.7)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(35.1)	(31.3)
Ending balance	351.3	342.5
Mortgage-Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	97.5	107.6
Transfers into Level 3	8.0	0.8
Transfers out of Level 3	(4.0)	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.3	0.0
Net unrealized investment gains (losses) included in OCI	0.1	5.0
Purchases	1.7	2.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(29.9)	(17.9)
Ending balance	73.7	97.5
Total Fixed Maturity Securities
Financial Assets
Beginning balance	514.0	423.3
Transfers into Level 3	19.8	77.3
Transfers out of Level 3	(12.4)	(3.7)
Total gains or losses
Net investment gains (losses) included in net income	0.3	0.0
Net unrealized investment gains (losses) included in OCI	3.7	(0.8)
Purchases	87.1	76.0
Issuances	0.0	0.0
Sales	(46.4)	(7.7)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(66.2)	(50.4)
Ending balance	499.9	514.0
Equity Securities & Limited Partnership Interests
Financial Assets
Beginning balance	4.5	2.0
Transfers into Level 3	28.9	2.1
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.6	0.4
Net unrealized investment gains (losses) included in OCI	0.0	0.0
Purchases	1.2	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(2.1)	0.0
Ending balance	$ 33.1	$ 4.5