Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Financial Assets
Beginning balance	$ 469.1	$ 533.0
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	$ 0.3	$ 5.8
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 gains (losses) included in OCI	$ (0.8)	$ 0.1
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent	Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent
Purchases	$ 19.3	$ 13.4
Issuances	0.0	0.0
Sales	(10.6)	(22.6)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(10.2)	(6.7)
Ending balance	467.1	523.0
Financial Liabilities
Beginning balance	73.1	75.5
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	1.2	1.9
Net unrealized gains (losses) included in OCI	$ 0.1	$ 0.2
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent	Other Comprehensive Income (Loss), Available-for-Sale Securities Adjustment, Net of Tax, Portion Attributable to Parent
Purchases	$ 0.0	$ 0.0
Issuances	0.7	1.4
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.1)	(4.7)
Ending balance	72.0	74.3
Total Fixed Maturity Securities
Financial Assets
Beginning balance	465.3	499.9
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	0.5	0.0
Net unrealized gains (losses) included in OCI	(0.8)	0.1
Purchases	19.3	13.4
Issuances	0.0	0.0
Sales	(10.6)	(22.6)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(10.2)	(6.7)
Ending balance	463.5	484.1
Municipal bonds
Financial Assets
Beginning balance	78.4	74.9
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 gains (losses) included in OCI	(0.2)	1.3
Purchases	9.1	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(2.1)	(0.1)
Ending balance	85.2	76.1
Corporate bonds
Financial Assets
Beginning balance	342.6	351.3
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	0.0	0.0
Net unrealized gains (losses) included in OCI	(2.1)	(0.6)
Purchases	10.2	13.4
Issuances	0.0	0.0
Sales	(9.8)	(22.4)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(4.3)	(0.8)
Ending balance	336.6	340.9
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	44.3	73.7
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	0.5	0.0
Net unrealized gains (losses) included in OCI	1.5	(0.6)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	(0.8)	(0.2)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(3.8)	(5.8)
Ending balance	41.7	67.1
Equity Securities & Limited Partnership Interests
Financial Assets
Beginning balance	3.8	33.1
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	(0.2)	5.8
Net unrealized gains (losses) included in OCI		0.0
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	0.0	0.0
Ending balance	$ 3.6	$ 38.9