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Accumulated Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  | $ 631.2 |  | $ 692.5 |  | $ 732.4
Other comprehensive income (loss), net of tax |  | 9.9 |  | 6.9 |  | 15.0
Ending balance |  | 652.2 |  | 631.2 |  | 692.5
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  | 2.8 |  | (21.4) |  | (19.8)
Less: income tax (expense) benefit | [1] | (1.0) |  | (1.2) |  | (0.9)
Net loss (gain) reclassified from AOCI to Net income (loss) |  | 3.2 |  | 3.7 |  | 3.0
Other comprehensive income (loss), net of tax |  | 8.2 |  | 24.2 |  | (1.6)
Ending balance |  | 11.0 |  | 2.8 |  | (21.4)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Unrealized gain (loss) on securities |  | 4.3 |  | 27.0 |  | 8.0
Less: income tax (expense) benefit |  | (1.0) |  | (6.5) |  | (1.9)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges |  | 3.3 |  | 20.5 |  | 6.1
Reclassification from AOCI | [2] | (1.1) |  |  |  | 0.4
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Unrealized gain (loss) on securities |  | 2.2 |  |  |  | (14.0)
Less: income tax (expense) benefit |  | (0.5) |  |  |  | 3.3
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges |  | 1.7 |  |  |  | (10.7)
Reclassification from AOCI | [2] | 5.3 |  | 4.9 |  | 3.5
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  | 0.4 | [3] | 17.7 | [3] | 1.1
Unrealized gain (loss) on securities |  | (10.7) |  | (12.2) |  | 63.6
Less: income tax (expense) benefit |  | 2.5 |  | 2.9 |  | (15.0)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges |  | (8.2) |  | (9.3) |  | 48.6
Reclassification from AOCI |  | 12.9 |  | (10.5) |  | (41.9)
Less: income tax (expense) benefit |  | (3.0) |  | 2.5 |  | 9.9
Net loss (gain) reclassified from AOCI to Net income (loss) |  | 9.9 |  | (8.0) |  | (32.0)
Other comprehensive income (loss), net of tax |  | 1.7 |  | (17.3) |  | 16.6
Ending balance | [3] | 2.1 |  | 0.4 |  | 17.7
Accumulated Other Comprehensive (Loss) Income |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  | 3.2 |  | (3.7) |  | (18.7)
Other comprehensive income (loss), net of tax |  | 9.9 |  | 6.9 |  | 15.0
Ending balance |  | $ 13.1 |  | $ 3.2 |  | $ (3.7)
[1] | Income tax amounts reclassified out of AOCI were included as a component of Income tax (provision) benefit.
[2] | Amounts amortized out of AOCI related to pension and other postretirement benefits were included within Net periodic postretirement benefit cost (see Note 5).
[3] | As of December 31, 2023, we estimate a net mark-to-market gain before tax of $2.2 million in AOCI will be reclassified into Net income (loss) upon settlement within the next 12 months.

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