Fair Value of Financial Instruments - Transfers Between Different Fair Value Levels (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Change in net unrealized investment gains (losses) on fixed maturity securities	Change in net unrealized investment gains (losses) on fixed maturity securities
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net investment losses	Net investment losses
Financial Assets
Beginning balance	$ 425.3	$ 371.4
Transfers into Level 3	79.4	193.8
Transfers out of Level 3	(3.7)	(42.8)
Total gains or losses
Net investment gains (losses) included in net income	0.4	(3.4)
Net unrealized investment gains (losses) included in OCI	(0.8)	(38.2)
Purchases	76.0	33.2
Issuances	0.0	0.0
Sales	(7.7)	(4.8)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(50.4)	(83.9)
Ending balance	518.5	425.3
Financial Liabilities
Beginning balance	91.3	111.4
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	$ 1.3	$ (14.8)
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Change in net unrealized investment gains (losses) on fixed maturity securities	Change in net unrealized investment gains (losses) on fixed maturity securities
Net unrealized investment gains (losses) included in OCI	$ 0.4	$ (2.6)
Purchases	0.0	0.0
Issuances	8.0	7.4
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(18.6)	(10.1)
Ending balance	82.4	91.3
Municipal bonds
Financial Assets
Beginning balance	54.4	60.8
Transfers into Level 3	0.0	0.6
Transfers out of Level 3	0.0	(3.2)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0
Net unrealized investment gains (losses) included in OCI	11.3	(10.5)
Purchases	9.5	0.2
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(1.2)	6.5
Ending balance	74.0	54.4
Corporate bonds
Financial Assets
Beginning balance	261.3	210.3
Transfers into Level 3	76.5	157.9
Transfers out of Level 3	(3.7)	(34.8)
Total gains or losses
Net investment gains (losses) included in net income	0.0	0.0
Net unrealized investment gains (losses) included in OCI	(17.1)	(16.1)
Purchases	64.5	20.2
Issuances	0.0	0.0
Sales	(7.7)	0.0
Settlements	0.0	0.0
Paydowns, maturities and distributions	(31.3)	(76.2)
Ending balance	342.5	261.3
Other asset-backed securities
Financial Assets
Beginning balance	107.6	98.9
Transfers into Level 3	0.8	34.5
Transfers out of Level 3	0.0	(4.8)
Total gains or losses
Net investment gains (losses) included in net income	0.0	(3.3)
Net unrealized investment gains (losses) included in OCI	5.0	(11.6)
Purchases	2.0	12.8
Issuances	0.0	0.0
Sales	0.0	(4.8)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(17.9)	(14.1)
Ending balance	97.5	107.6
Total
Financial Assets
Beginning balance	423.3	370.0
Transfers into Level 3	77.3	193.0
Transfers out of Level 3	(3.7)	(42.8)
Total gains or losses
Net investment gains (losses) included in net income	0.0	(3.3)
Net unrealized investment gains (losses) included in OCI	(0.8)	(38.2)
Purchases	76.0	33.2
Issuances	0.0	0.0
Sales	(7.7)	(4.8)
Settlements	0.0	0.0
Paydowns, maturities and distributions	(50.4)	(83.8)
Ending balance	514.0	423.3
Equity securities
Financial Assets
Beginning balance	2.0	1.4
Transfers into Level 3	2.1	0.8
Transfers out of Level 3	0.0	0.0
Total gains or losses
Net investment gains (losses) included in net income	0.4	(0.1)
Net unrealized investment 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	(0.1)
Ending balance	$ 4.5	$ 2.0

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