Fair Value of Financial Instruments - Rollforward of Instruments Measured on Recurring Basis (Details) - Level 3 - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
|
| Financial Assets | ||||
| Beginning balance | $ 467.1 | $ 523.0 | $ 469.1 | $ 533.0 |
| 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 | $ 0.0 | $ (0.9) | $ 0.2 | $ 4.9 |
| Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Net investment losses | Net investment losses | Net investment losses | Net investment losses |
| Net unrealized gains (losses) included in OCI | $ (7.0) | $ 2.8 | $ (7.7) | $ 3.0 |
| 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 | $ 13.9 | $ 8.9 | $ 33.1 | $ 22.3 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | (11.6) | (4.7) | (22.2) | (27.1) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (18.2) | (10.1) | (28.3) | (17.1) |
| Ending balance | 444.2 | 519.0 | 444.2 | 519.0 |
| Financial Liabilities | ||||
| Beginning balance | 72.0 | 74.3 | 73.1 | 75.5 |
| 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 related to financial liabilities | 3.7 | 3.7 | 4.9 | 5.6 |
| Net unrealized gains (losses) included in OCI | $ (0.3) | $ (0.4) | $ (0.2) | $ (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 | 0.8 | 0.7 | 1.5 | 2.1 |
| Sales | 0.0 | 0.0 | 0.0 | 0.0 |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (3.9) | (4.3) | (7.0) | (9.0) |
| Ending balance | 72.3 | 74.0 | 72.3 | 74.0 |
| Total Fixed Maturity Securities | ||||
| Financial Assets | ||||
| Beginning balance | 463.5 | 484.1 | 465.3 | 499.9 |
| 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 | 0.4 | 0.0 |
| Net unrealized gains (losses) included in OCI | (7.0) | 2.8 | (7.7) | 3.0 |
| Purchases | 13.9 | 8.9 | 33.1 | 22.3 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | (11.6) | (4.7) | (22.2) | (27.1) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (18.2) | (10.1) | (28.3) | (17.1) |
| Ending balance | 440.6 | 481.0 | 440.6 | 481.0 |
| Municipal bonds | ||||
| Financial Assets | ||||
| Beginning balance | 85.2 | 76.1 | 78.4 | 74.9 |
| 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 | (0.4) | 0.1 | (0.5) | 1.5 |
| Purchases | 0.0 | 0.0 | 9.0 | 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 | (0.4) | (0.3) | (2.5) | (0.5) |
| Ending balance | 84.4 | 75.9 | 84.4 | 75.9 |
| Corporate bonds | ||||
| Financial Assets | ||||
| Beginning balance | 336.6 | 340.9 | 342.6 | 351.3 |
| 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 | 0.0 | 0.0 | 0.0 | 0.0 |
| Net unrealized gains (losses) included in OCI | (3.3) | 2.4 | (5.4) | 1.8 |
| Purchases | 13.9 | 8.9 | 24.1 | 22.3 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | (11.6) | (3.0) | (21.5) | (25.3) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (13.7) | (7.3) | (17.9) | (8.1) |
| Ending balance | 321.9 | 342.0 | 321.9 | 342.0 |
| Mortgage Backed and Other Asset- Backed Securities | ||||
| Financial Assets | ||||
| Beginning balance | 41.7 | 67.1 | 44.3 | 73.7 |
| 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 | 0.4 | 0.0 |
| Net unrealized gains (losses) included in OCI | (3.3) | 0.3 | (1.8) | (0.2) |
| Purchases | 0.0 | 0.0 | 0.0 | 0.0 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | 0.0 | (1.7) | (0.7) | (1.8) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (4.1) | (2.5) | (7.9) | (8.5) |
| Ending balance | 34.3 | 63.2 | 34.3 | 63.2 |
| Equity Securities | ||||
| Financial Assets | ||||
| Beginning balance | 3.6 | 38.9 | 3.8 | 33.1 |
| 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 | (0.9) | (0.2) | 4.9 |
| Net unrealized gains (losses) included in OCI | 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 | 0.0 | 0.0 |
| Ending balance | $ 3.6 | $ 38.0 | $ 3.6 | $ 38.0 |