Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Total gains or losses
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net investment gains (losses)	Net investment gains (losses)
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
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
Level 3
Financial Assets
Beginning balance	$ 533.0	$ 518.5
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(4.4)
Total gains or losses
Net investment gains (losses) included in net income	5.8	0.1
Net unrealized gains (losses) included in OCI	0.1	1.1
Purchases	13.4	14.1
Issuances	0.0	0.0
Sales	(22.6)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(6.7)	(19.5)
Ending balance	523.0	509.9
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	1.9	1.3
Net unrealized gains (losses) included in OCI	0.2	(0.1)
Purchases	0.0	0.0
Issuances	1.4	1.4
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(4.7)	(4.2)
Ending balance	74.3	80.8
Level 3 | Municipal bonds
Financial Assets
Beginning balance	74.9	74.0
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	0.0	0.0
Net unrealized gains (losses) included in OCI	1.3	(0.7)
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(0.1)	(0.1)
Ending balance	76.1	73.2
Level 3 | Corporate bonds
Financial Assets
Beginning balance	351.3	342.5
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(4.4)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0
Net unrealized gains (losses) included in OCI	(0.6)	(0.6)
Purchases	13.4	14.1
Issuances	0.0	0.0
Sales	(22.4)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(0.8)	(6.1)
Ending balance	340.9	345.5
Level 3 | Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	73.7	97.5
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	0.0	0.0
Net unrealized gains (losses) included in OCI	(0.6)	2.4
Purchases	0.0	0.0
Issuances	0.0	0.0
Sales	(0.2)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(5.8)	(11.2)
Ending balance	67.1	88.7
Level 3 | Total Fixed Maturity Securities
Financial Assets
Beginning balance	499.9	514.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(4.4)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0
Net unrealized gains (losses) included in OCI	0.1	1.1
Purchases	13.4	14.1
Issuances	0.0	0.0
Sales	(22.6)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(6.7)	(17.4)
Ending balance	484.1	507.4
Level 3 | Equity Securities & Limited Partnership Interests
Financial Assets
Beginning balance	33.1	4.5
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	5.8	0.1
Net unrealized gains (losses) included in OCI	0.0	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	(2.1)
Ending balance	$ 38.9	$ 2.5