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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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