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Income Taxes - Changes in Valuation Allowance (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Movement in Valuation Allowances and Reserves [Roll Forward] |  |  |
Balance, valuation allowance | $ 487.9 | $ 485.8 | $ 499.4
Remeasurement of deferred tax assets | 0.0 | 0.0 | 0.0
Release of valuation allowance | 0.0 | 0.0 | 0.0
Expiration of net operating losses | (7.2) | (15.4) | (13.2)
Other change in valuation allowance | 56.9 | 17.5 | (0.4)
Balance, valuation allowance | $ 537.6 | $ 487.9 | $ 485.8

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