Stock-Based Compensation - Employee Stock Option and Incentive Plan Activity Additional Information (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Additional information
Intrinsic value of options exercised	$ 0.8	$ 0.0	$ 0.0
Weighted average grant-date fair value (in dollars per share)	$ 8.08	$ 2.08	$ 6.60
Fair value of options vested	$ 2.4	$ 4.9	$ 8.0
Intrinsic value of options outstanding	24.7
Intrinsic value of options vested and expected to vest	23.3
Compensation cost not yet recognized	$ 11.3
Weighted average period compensation cost is expected to be recognized (in years)	0 years

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for option under share-based payment arrangement.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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