Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Financial Assets
Beginning balance	$ 526.6	$ 478.8	$ 518.5	$ 425.3
Transfers into Level 3	19.8	0.0	19.8	36.9
Transfers out of Level 3	(7.9)	(3.7)	(12.3)	(3.7)
Total gains or losses
Net investment gains (losses) included in net income	(0.4)	0.2	(0.1)	0.2
Net unrealized gains (losses) included in OCI	(0.5)	(4.9)	(0.6)	(6.7)
Purchases	24.3	19.2	67.8	54.6
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(7.7)
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(13.8)	(12.0)	(45.0)	(21.3)
Ending balance	548.1	477.6	548.1	477.6
Financial Liabilities
Beginning balance	77.9	83.6	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	6.6	(3.2)	7.6	(5.6)
Net unrealized gains (losses) included in OCI	0.2	0.4	0.2	(0.3)
Purchases	0.0	0.0	0.0	0.0
Issuances	0.8	2.0	3.6	6.3
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(4.4)	(4.5)	(12.7)	(13.4)
Ending balance	$ 81.1	$ 78.3	$ 81.1	$ 78.3
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	$ 0.4	$ (0.2)	$ (0.1)	$ (0.2)
Municipal bonds
Financial Assets
Beginning balance	72.1	63.9	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	1.4	5.9	0.0	6.5
Purchases	3.2	0.0	3.2	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	(0.1)	(0.1)	(0.6)	(0.7)
Ending balance	76.6	69.7	76.6	69.7
Corporate bonds
Financial Assets
Beginning balance	366.4	311.5	342.5	261.3
Transfers into Level 3	11.8	0.0	11.8	36.1
Transfers out of Level 3	(3.9)	(3.7)	(8.3)	(3.7)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0	(0.8)	0.0
Net unrealized gains (losses) included in OCI	(3.4)	(12.8)	(3.6)	(14.1)
Purchases	19.9	17.5	61.6	43.2
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(7.7)
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(8.0)	(7.3)	(20.4)	(9.9)
Ending balance	382.8	305.2	382.8	305.2
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	85.4	101.4	97.5	107.6
Transfers into Level 3	8.0	0.0	8.0	0.8
Transfers out of Level 3	(4.0)	0.0	(4.0)	0.0
Total gains or losses
Net investment gains (losses) included in net income	(0.5)	0.0	0.3	0.0
Net unrealized gains (losses) included in OCI	1.5	2.0	3.0	0.9
Purchases	0.0	1.7	1.8	1.9
Issuances	0.0	0.0	0.0
Sales	0.0	0.0	0.0
Settlements	0.0	0.0	0.0
Paydowns, maturities and distributions	(5.7)	(4.6)	(21.9)	(10.7)
Ending balance	84.7	100.5	84.7	100.5
Total Fixed Maturity Securities
Financial Assets
Beginning balance	523.9	476.8	514.0	423.3
Transfers into Level 3	19.8	0.0	19.8	36.9
Transfers out of Level 3	(7.9)	(3.7)	(12.3)	(3.7)
Total gains or losses
Net investment gains (losses) included in net income	(0.5)	0.0	(0.5)	0.0
Net unrealized gains (losses) included in OCI	(0.5)	(4.9)	(0.6)	(6.7)
Purchases	23.1	19.2	66.6	54.6
Issuances	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(7.7)
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(13.8)	(12.0)	(42.9)	(21.3)
Ending balance	544.1	475.4	544.1	475.4
Equity securities
Financial Assets
Beginning balance	2.7	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	0.1	0.2	0.4	0.2
Net unrealized gains (losses) included in OCI	0.0	0.0	0.0	0.0
Purchases	1.2	0.0	1.2	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	$ 4.0	$ 2.2	$ 4.0	$ 2.2