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Stock Options - Additional Information (Details) - USD ($) $ in Millions		6 Months Ended
	Mar. 12, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	May 31, 2018	Dec. 31, 2016	Apr. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options outstanding		8,105,482	7,763,464
Stock options outstanding		998,177
Unrecognized compensation cost		$ 25.0
Weighted-average period for recognition of compensation cost		3 years 2 months 8 days
Intrinsic value of options exercised		$ 4.6		$ 0.6
2006 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Remaining shares available to be granted						0
Options outstanding		1,397,203
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options outstanding		6,260,520
Common stock reserved for issuance					8,211,980		5,270,095
2019 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	4,750,000
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	4.00%
Maximum shares of common stock may be issued	5,000,000
Stock options outstanding		447,759
2019 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	525,000
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock	1.00%
Maximum shares allowed to be issued under ESPP	5,250,000
Purchase of common stock through payroll deductions expressed in percentage of fair market value	85.00%
Purchase rights outstanding under the ESPP		0

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Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock.

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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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Unrecognized cost of unvested options awarded to employees as compensation.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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