Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Financial Assets
Beginning balance	$ 519.0	$ 526.6	$ 533.0	$ 518.5
Transfers into Level 3	1.2	19.8	1.2	19.8
Transfers out of Level 3	0.0	(7.9)	0.0	(12.3)
Total gains or losses
Net investment gains (losses) included in net income	$ (0.3)	$ (0.4)	$ 4.6	$ (0.1)
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)
Net unrealized gains (losses) included in OCI	$ 11.3	$ (0.5)	$ 14.4	$ (0.6)
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
Purchases	$ 15.7	$ 24.3	$ 37.9	$ 67.8
Issuances	0.0	0.0	0.0	0.0
Sales	(28.9)	0.0	(56.2)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(2.9)	(13.8)	(19.8)	(45.0)
Ending balance	515.1	548.1	515.1	548.1
Financial Liabilities
Beginning balance	74.0	77.9	75.5	82.4
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 related to financial liabilities	(1.2)	6.6	4.4	7.6
Net unrealized gains (losses) included in OCI	$ (0.2)	$ 0.2	$ (0.4)	$ 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	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	$ 0.0	$ 0.0
Issuances	1.0	0.8	3.1	3.6
Sales	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(3.8)	(4.4)	(12.8)	(12.7)
Ending balance	69.8	81.1	69.8	81.1
Municipal bonds
Financial Assets
Beginning balance	75.9	72.1	74.9	74.0
Transfers into Level 3	1.2	0.0	1.2	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	1.1	1.4	2.6	0.0
Purchases	0.0	3.2	0.0	3.2
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)	(0.6)
Ending balance	78.1	76.6	78.1	76.6
Corporate bonds
Financial Assets
Beginning balance	342.0	366.4	351.3	342.5
Transfers into Level 3	0.0	11.8	0.0	11.8
Transfers out of Level 3	0.0	(3.9)	0.0	(8.3)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0	0.0	(0.8)
Net unrealized gains (losses) included in OCI	9.9	(3.4)	11.8	(3.6)
Purchases	11.7	19.9	33.9	61.6
Issuances	0.0	0.0	0.0	0.0
Sales	(23.6)	0.0	(49.0)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	0.2	(8.0)	(7.8)	(20.4)
Ending balance	340.2	382.8	340.2	382.8
Mortgage Backed and Other Asset- Backed Securities
Financial Assets
Beginning balance	63.2	85.4	73.7	97.5
Transfers into Level 3	0.0	8.0	0.0	8.0
Transfers out of Level 3	0.0	(4.0)	0.0	(4.0)
Total gains or losses
Net investment gains (losses) included in net income	0.0	(0.5)	0.0	0.3
Net unrealized gains (losses) included in OCI	0.3	1.5	0.0	3.0
Purchases	4.0	0.0	4.0	1.8
Issuances	0.0	0.0	0.0	0.0
Sales	(5.3)	0.0	(7.2)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(3.1)	(5.7)	(11.4)	(21.9)
Ending balance	59.1	84.7	59.1	84.7
Total Fixed Maturity Securities
Financial Assets
Beginning balance	481.0	523.9	499.9	514.0
Transfers into Level 3	1.2	19.8	1.2	19.8
Transfers out of Level 3	0.0	(7.9)	0.0	(12.3)
Total gains or losses
Net investment gains (losses) included in net income	0.0	(0.5)	0.0	(0.5)
Net unrealized gains (losses) included in OCI	11.3	(0.5)	14.4	(0.6)
Purchases	15.7	23.1	37.9	66.6
Issuances	0.0	0.0	0.0	0.0
Sales	(28.9)	0.0	(56.2)	0.0
Settlements	0.0	0.0	0.0	0.0
Paydowns, maturities and distributions	(2.9)	(13.8)	(19.8)	(42.9)
Ending balance	477.4	544.1	477.4	544.1
Equity Securities & Limited Partnership Interests
Financial Assets
Beginning balance	38.0	2.7	33.1	4.5
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	(0.3)	0.1	4.6	0.4
Net unrealized gains (losses) included in OCI		0.0	0.0	0.0
Purchases	0.0	1.2	0.0	1.2
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	0.0	(2.1)
Ending balance	$ 37.7	$ 4.0	$ 37.7	$ 4.0