Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Financial Assets
Beginning balance	$ 518.5	$ 425.3
Transfers into Level 3	0.0	6.3
Transfers out of Level 3	(4.4)	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.1	0.0
Net unrealized gains (losses) included in OCI	1.1	1.7
Purchases	14.1	19.2
Issuances	0.0	0.0
Sales	0.0	(2.6)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(19.5)	(3.9)
Ending balance	509.9	446.0
Financial Liabilities
Beginning balance	82.4	91.0
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 related to financial liabilities	(1.3)	(0.2)
Net unrealized gains (losses) included in OCI	(0.1)	0.0
Purchases	0.0	0.0
Issuances	1.4	2.1
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(4.2)	(3.4)
Ending balance	$ 80.8	$ 89.9
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
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 gains (losses) included in OCI	(0.7)	1.2
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)	(0.2)
Ending balance	73.2	55.4
Corporate bonds
Financial Assets
Beginning balance	342.5	261.3
Transfers into Level 3	0.0	5.9
Transfers out of Level 3	(4.4)	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)	0.9
Purchases	14.1	19.0
Issuances	0.0	0.0
Sales	0.0	(2.6)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(6.1)	(0.5)
Ending balance	345.5	284.0
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	97.5	107.6
Transfers into Level 3	0.0	0.4
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	2.4	(0.4)
Purchases	0.0	0.2
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(11.2)	(3.2)
Ending balance	88.7	104.6
Total Fixed Maturity Securities
Financial Assets
Beginning balance	514.0	423.3
Transfers into Level 3	0.0	6.3
Transfers out of Level 3	(4.4)	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	1.1	1.7
Purchases	14.1	19.2
Issuances	0.0	0.0
Sales	0.0	(2.6)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(17.4)	(3.9)
Ending balance	507.4	444.0
Equity securities
Financial Assets
Beginning balance	4.5	2.0
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	0.1	0.0
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	(2.1)	0.0
Ending balance	$ 2.5	$ 2.0

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