Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions		3 Months Ended
Mar. 31, 2021	Mar. 31, 2021	Mar. 31, 2020
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility (as a percent)		55.60%	53.90%
Risk-free interest rate (as a percent)		1.02%	1.71%
Expected term (years)		6 years 2 months 12 days	6 years 3 months 18 days
Compensation expense
Unamortized compensation expense related to unvested options	$ 71.4	$ 71.4
Period to recognize unamortized compensation expense	3 years
ESPP
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)		50.80%
Expected volatility, high end of range (as a percent)		66.40%
Expected volatility (as a percent)			50.80%
Risk-free interest rate, low end of range (as a percent)		0.09%	1.56%
Risk-free interest rate, high end of range (as a percent)		1.65%	1.65%
Compensation expense
Unamortized compensation expense related to unvested options	$ 0.7	$ 0.7
Period to recognize unamortized compensation expense	8 months 12 days
ESPP \| Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)		6 months	6 months
ESPP \| Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)		2 years	2 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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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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Expected term of 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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