Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Common stock fair value per share minimum	21.80	19.61	11.50
Common stock fair value per share maximum	43.16	25.67	26.76
Expected volatility, low end of range (as a percent)	70.97%	77.42%	75.77%
Expected volatility, high end of range (as a percent)	73.10%	96.73%	90.83%
Risk-free interest rate, low end of range (as a percent)	2.29%	0.96%	1.03%
Risk-free interest rate, high end of range (as a percent)	3.10%	2.37%	2.18%
Compensation expense
Unamortized compensation expense related to unvested options	$ 42.8	$ 30.7
Period to recognize unamortized compensation expense	2 years 8 months 16 days
Employee stock options \| Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	5 years 2 months 23 days	5 years 2 months 23 days	5 years 2 months 23 days
Employee stock options \| Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 years 29 days	6 years 29 days	6 years 29 days
ESPP
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)	57.00%	67.80%	67.80%
Expected volatility, high end of range (as a percent)	71.40%	79.80%	79.80%
Risk-free interest rate, low end of range (as a percent)	1.47%	0.47%	0.47%
Risk-free interest rate, high end of range (as a percent)	2.70%	1.80%	0.93%
Compensation expense
Unamortized compensation expense related to unvested options	$ 0.8	$ 1.1
Period to recognize unamortized compensation expense	11 months 6 days
ESPP \| Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 months	6 months	6 months
ESPP \| Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	2 years	2 years	2 years

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

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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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- Definition

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 share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

The estimated fair value of the entity's common stock at the date of grant of stock options, used as an input in calculating the grant date fair value of the options maximum.

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The minimum estimated fair value of the entity's common stock at the date of grant of stock options, used as an input in calculating the grant date fair value of the options.

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