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Accumulated Other Comprehensive Loss - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Sep. 30, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Sep. 30, 2023 | Sep. 30, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | $ 638.1 |  | $ 635.6 | $ 631.2 | $ 650.0 |  | $ 708.2 | $ 692.5 | $ 631.2 |  | $ 692.5 |
Other comprehensive income (loss), net of tax |  | 6.0 |  | (7.3) | (1.1) | (0.4) |  | (35.6) | 18.0 | (2.4) |  | (18.0) |
Ending balance |  | 640.7 |  | 638.1 | 635.6 | 642.0 |  | 650.0 | 708.2 | 640.7 |  | 642.0 |
Defined Benefit Plans: |  |  |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | (1.1) |  |  | 2.8 | (19.6) |  |  | (21.4) | 2.8 |  | (21.4) |
Less: income tax (expense) benefit | [1] | (0.3) |  |  |  | (0.2) |  |  |  | (0.7) |  | (0.9) |
Net loss (gain) reclassified from AOCI to Net income (loss) |  | 0.8 |  |  |  | 1.0 |  |  |  | 2.4 |  | 2.8 |
Other comprehensive income (loss), net of tax |  | 0.8 |  |  |  | 1.0 |  |  |  | (3.1) |  | 2.8 |
Ending balance |  | (0.3) |  | (1.1) |  | (18.6) |  | (19.6) |  | (0.3) |  | (18.6) |
Defined Benefit Plans: Net Actuarial Loss |  |  |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Unrealized (loss) gain on cash flow hedges |  |  |  |  |  |  |  |  |  | (0.6) |  |  |
Less: income tax benefit |  |  |  |  |  |  |  |  |  | 0.1 |  |  |
Net unrealized (loss) gain on cash flow hedges |  |  |  |  |  |  |  |  |  | (0.5) |  |  |
Reclassification of unrealized loss (gain) upon settlement of cash flow hedges | [2] | (0.3) |  |  |  |  |  |  |  | (0.9) |  |  |
Defined Benefit Plans: Net Prior Service Cost |  |  |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Unrealized (loss) gain on cash flow hedges |  |  |  |  |  |  |  |  |  | (6.6) |  |  |
Less: income tax benefit |  |  |  |  |  |  |  |  |  | 1.6 |  |  |
Net unrealized (loss) gain on cash flow hedges |  |  |  |  |  |  |  |  |  | (5.0) |  |  |
Reclassification of unrealized loss (gain) upon settlement of cash flow hedges | [2] | 1.4 |  |  |  | 1.2 |  |  |  | 4.0 |  | 3.7 |
Cash Flow Hedges: |  |  |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | (4.1) |  |  | 0.4 | (1.7) |  |  | 17.7 | 0.4 |  | 17.7 |
Unrealized (loss) gain on cash flow hedges |  | 2.4 |  |  |  | (9.8) |  |  |  | (9.2) |  | (18.2) |
Less: income tax benefit |  | (0.6) |  |  |  | 2.2 |  |  |  | 2.1 |  | 4.2 |
Net unrealized (loss) gain on cash flow hedges |  | 1.8 |  |  |  | (7.6) |  |  |  | (7.1) |  | (14.0) |
Reclassification of unrealized loss (gain) upon settlement of cash flow hedges |  | 4.5 |  |  |  | 8.1 |  |  |  | 10.2 |  | (8.9) |
Less: income tax (expense) benefit | [1] | (1.1) |  |  |  | (1.9) |  |  |  | (2.4) |  | 2.1 |
Net loss (gain) reclassified from AOCI to Net income (loss) |  | 3.4 |  |  |  | 6.2 |  |  |  | 7.8 |  | (6.8) |
Other comprehensive income (loss), net of tax |  | 5.2 |  |  |  | (1.4) |  |  |  | 0.7 |  | (20.8) |
Ending balance |  | 1.1 | [3] | (4.1) |  | (3.1) | [3] | (1.7) |  | 1.1 | [3] | (3.1) | [3]
Accumulated Other Comprehensive Loss |  |  |  |  |  |  |  |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | (5.2) |  | 2.1 | 3.2 | (21.3) |  | 14.3 | (3.7) | 3.2 |  | (3.7) |
Other comprehensive income (loss), net of tax |  | 6.0 |  | (7.3) | (1.1) | (0.4) |  | (35.6) | 18.0 |  |  |  |
Ending balance |  | $ 0.8 |  | $ (5.2) | $ 2.1 | $ (21.7) |  | $ (21.3) | $ 14.3 | $ 0.8 |  | $ (21.7) |
[1] | Income tax amounts reclassified out of AOCI were included as a component of Income tax provision.
[2] | Amounts amortized out of AOCI related to pension and other postretirement benefits were included within Net periodic postretirement benefit cost (see Note 3).
[3] | As of September 30, 2023, we estimate a net mark-to-market gain before tax of $1.0 million in AOCI will be reclassified into Net income (loss) upon settlement within the next 12 months.

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