Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Financial Assets
Beginning balance	$ 509.9	$ 446.0	$ 518.5	$ 425.3
Transfers into Level 3	0.0	30.6	0.0	36.9
Transfers out of Level 3	0.0	0.0	(4.4)	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.2	0.0	0.3	0.0
Net unrealized gains (losses) included in OCI	(1.2)	(3.7)	(0.1)	(1.8)
Purchases	29.4	16.1	43.5	35.4
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(5.1)	0.0	(7.7)
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(11.7)	(5.1)	(31.2)	(9.3)
Ending balance	526.6	478.8	526.6	478.8
Financial Liabilities
Beginning balance	80.8	91.2	82.4	91.3
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	0.3	3.4	(1.0)	2.5
Net unrealized gains (losses) included in OCI	0.1	(0.9)	0.0	(0.6)
Purchases	0.0	0.0	0.0	0.0
Issuances	1.4	2.2	2.8	4.3
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(4.1)	(5.5)	(8.3)	(8.9)
Ending balance	$ 77.9	$ 83.6	$ 77.9	$ 83.6
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)	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	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	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
Total gains or losses
Net investment (gains) losses included in net income related to financial liabilities	$ (0.2)	$ 0.0	$ (0.3)	$ 0.0
Municipal bonds
Financial Assets
Beginning balance	73.2	55.4	74.0	54.4
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0	0.0	0.0
Net unrealized gains (losses) included in OCI	(0.7)	(0.8)	(1.4)	0.5
Purchases	0.0	9.5	0.0	9.5
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(0.4)	(0.2)	(0.5)	(0.5)
Ending balance	72.1	63.9	72.1	63.9
Corporate bonds
Financial Assets
Beginning balance	345.5	284.0	342.5	261.3
Transfers into Level 3	0.0	30.2	0.0	36.1
Transfers out of Level 3	0.0	0.0	(4.4)	0.0
Total gains or losses
Net investment gains (losses) included in net income	(0.8)	0.0	(0.8)	0.0
Net unrealized gains (losses) included in OCI	0.4	(2.2)	(0.2)	(1.2)
Purchases	27.6	6.6	41.7	25.6
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(5.1)	0.0	(7.7)
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(6.3)	(2.0)	(12.4)	(2.6)
Ending balance	366.4	311.5	366.4	311.5
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	88.7	104.6	97.5	107.6
Transfers into Level 3	0.0	0.4	0.0	0.8
Transfers out of Level 3	0.0	0.0	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.8	0.0	0.8	0.0
Net unrealized gains (losses) included in OCI	(0.9)	(0.7)	1.5	(1.1)
Purchases	1.8	0.0	1.8	0.3
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(5.0)	(2.9)	(16.2)	(6.2)
Ending balance	85.4	101.4	85.4	101.4
Total Fixed Maturity Securities
Financial Assets
Beginning balance	507.4	444.0	514.0	423.3
Transfers into Level 3	0.0	30.6	0.0	36.9
Transfers out of Level 3	0.0	0.0	(4.4)	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0	0.0	0.0
Net unrealized gains (losses) included in OCI	(1.2)	(3.7)	(0.1)	(1.8)
Purchases	29.4	16.1	43.5	35.4
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	(5.1)	0.0	(7.7)
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(11.7)	(5.1)	(29.1)	(9.3)
Ending balance	523.9	476.8	523.9	476.8
Equity securities
Financial Assets
Beginning balance	2.5	2.0	4.5	2.0
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.2	0.0	0.3	0.0
Net unrealized gains (losses) included in OCI	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	0.0	0.0	(2.1)	0.0
Ending balance	$ 2.7	$ 2.0	$ 2.7	$ 2.0