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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

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