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Income Taxes - Changes in Valuation Allowance (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance, valuation allowance $ 492.4 $ 552.5 $ 607.8
Remeasurement of deferred tax assets 0.0 (41.6) (32.1)
Release of valuation allowance 0.0 0.0 0.0
Expiration of net operating losses (11.7) (10.8) (12.3)
Other change in valuation allowance 18.7 (7.7) (11.0)
Balance, valuation allowance $ 499.4 $ 492.4 $ 552.5