Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Financial Assets
Beginning balance	$ 533.0	$ 518.5
Transfers into Level 3	1.2	48.7
Transfers out of Level 3	0.0	(12.4)
Total gains or losses
Net investment gains (losses) included in net income	$ (1.5)	$ 0.9
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net investment losses	Net investment losses
Net unrealized investment gains (losses) included in OCI	$ 12.8	$ 3.7
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
Purchases	$ 57.1	$ 88.3
Issuances	0.0	0.0
Sales	(108.6)	(46.4)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(24.9)	(68.3)
Ending balance	469.1	533.0
Financial Liabilities
Beginning balance	75.5	82.4
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 related to financial liabilities	10.8	5.4
Net unrealized gains (losses) included in OCI	$ (0.4)	$ 0.0
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
Purchases	$ 0.0	$ 0.0
Issuances	4.3	5.4
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(17.1)	(17.7)
Ending balance	73.1	75.5
Municipal bonds
Financial Assets
Beginning balance	74.9	74.0
Transfers into Level 3	1.2	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	2.8	(2.1)
Purchases	0.7	4.2
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	78.4	74.9
Corporate bonds
Financial Assets
Beginning balance	351.3	342.5
Transfers into Level 3	0.0	11.8
Transfers out of Level 3	0.0	(8.4)
Total gains or losses
Net investment gains (losses) included in net income	(2.0)	0.0
Net unrealized investment gains (losses) included in OCI	7.6	5.7
Purchases	52.4	81.2
Issuances	0.0	0.0
Sales	(57.0)	(46.4)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(9.7)	(35.1)
Ending balance	342.6	351.3
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	73.7	97.5
Transfers into Level 3	0.0	8.0
Transfers out of Level 3	0.0	(4.0)
Total gains or losses
Net investment gains (losses) included in net income	(5.9)	0.3
Net unrealized investment gains (losses) included in OCI	2.4	0.1
Purchases	4.0	1.7
Issuances	0.0	0.0
Sales	(16.6)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(13.3)	(29.9)
Ending balance	44.3	73.7
Total Fixed Maturity Securities
Financial Assets
Beginning balance	499.9	514.0
Transfers into Level 3	1.2	19.8
Transfers out of Level 3	0.0	(12.4)
Total gains or losses
Net investment gains (losses) included in net income	(7.9)	0.3
Net unrealized investment gains (losses) included in OCI	12.8	3.7
Purchases	57.1	87.1
Issuances	0.0	0.0
Sales	(73.6)	(46.4)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(24.2)	(66.2)
Ending balance	465.3	499.9
Equity Securities & Limited Partnership Interests
Financial Assets
Beginning balance	33.1	4.5
Transfers into Level 3	0.0	28.9
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	6.4	0.6
Net unrealized investment gains (losses) included in OCI		0.0
Purchases		1.2
Issuances		0.0
Sales	(35.0)	0.0
Settlements		0.0
Paydowns, maturities and distributions	(0.7)	(2.1)
Ending balance	$ 3.8	$ 33.1