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, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Financial Assets | ||||
| Beginning balance | $ 444.2 | $ 519.0 | $ 469.1 | $ 533.0 |
| Transfers into Level 3 | 0.0 | 1.2 | 0.0 | 1.2 |
| 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 | $ (3.0) | $ (0.3) | $ (2.8) | $ 4.6 |
| 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) |
| Net unrealized gains (losses) included in OCI | $ (16.4) | $ 11.3 | $ (24.2) | $ 14.4 |
| 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 | $ 26.0 | $ 15.7 | $ 59.2 | $ 37.9 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | (8.5) | (28.9) | (30.7) | (56.2) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (14.3) | (2.9) | (42.6) | (19.8) |
| Ending balance | 428.0 | 515.1 | 428.0 | 515.1 |
| Financial Liabilities | ||||
| Beginning balance | 72.3 | 74.0 | 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 | (2.4) | (1.2) | 2.5 | 4.4 |
| Net unrealized gains (losses) included in OCI | $ (0.2) | $ (0.2) | $ (0.4) | $ (0.4) |
| 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 | 1.0 | 2.3 | 3.1 |
| Sales | 0.0 | 0.0 | 0.0 | 0.0 |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (4.1) | (3.8) | (11.1) | (12.8) |
| Ending balance | 66.4 | 69.8 | 66.4 | 69.8 |
| Total Fixed Maturity Securities | ||||
| Financial Assets | ||||
| Beginning balance | 440.6 | 481.0 | 465.3 | 499.9 |
| Transfers into Level 3 | 0.0 | 1.2 | 0.0 | 1.2 |
| 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 | (3.0) | 0.0 | (2.6) | 0.0 |
| Net unrealized gains (losses) included in OCI | (16.4) | 11.3 | (24.2) | 14.4 |
| Purchases | 26.0 | 15.7 | 59.2 | 37.9 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | (8.5) | (28.9) | (30.7) | (56.2) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (14.3) | (2.9) | (42.6) | (19.8) |
| Ending balance | 424.4 | 477.4 | 424.4 | 477.4 |
| Municipal bonds | ||||
| Financial Assets | ||||
| Beginning balance | 84.4 | 75.9 | 78.4 | 74.9 |
| Transfers into Level 3 | 0.0 | 1.2 | 0.0 | 1.2 |
| Transfers out of Level 3 | 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) | 1.1 | (3.9) | 2.6 |
| Purchases | 0.0 | 0.0 | 9.1 | 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 | (0.6) | (0.1) | (3.1) | (0.6) |
| Ending balance | 80.5 | 78.1 | 80.5 | 78.1 |
| Corporate bonds | ||||
| Financial Assets | ||||
| Beginning balance | 321.9 | 342.0 | 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 | (2.9) | 0.0 | (2.9) | 0.0 |
| Net unrealized gains (losses) included in OCI | (12.0) | 9.9 | (17.4) | 11.8 |
| Purchases | 26.0 | 11.7 | 50.1 | 33.9 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | (8.5) | (23.6) | (30.0) | (49.0) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (11.0) | 0.2 | (28.9) | (7.8) |
| Ending balance | 313.5 | 340.2 | 313.5 | 340.2 |
| Mortgage Backed and Other Asset- Backed Securities | ||||
| Financial Assets | ||||
| Beginning balance | 34.3 | 63.2 | 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 | (0.1) | 0.0 | 0.3 | 0.0 |
| Net unrealized gains (losses) included in OCI | (1.1) | 0.3 | (2.9) | 0.0 |
| Purchases | 0.0 | 4.0 | 0.0 | 4.0 |
| Issuances | 0.0 | 0.0 | 0.0 | 0.0 |
| Sales | 0.0 | (5.3) | (0.7) | (7.2) |
| Settlements | 0.0 | 0.0 | 0.0 | 0.0 |
| Paydowns, maturities and distributions | (2.7) | (3.1) | (10.6) | (11.4) |
| Ending balance | 30.4 | 59.1 | 30.4 | 59.1 |
| Equity Securities | ||||
| Financial Assets | ||||
| Beginning balance | 3.6 | 38.0 | 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 | (0.3) | (0.2) | 4.6 |
| 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 | $ 37.7 | $ 3.6 | $ 37.7 |