Stock Option Plan - Exercise Price Range (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Information about stock options outstanding
Options Outstanding, Number	5,643,410	3,778,259
Options Outstanding, Weighted Average Remaining Contractual Life	8 years 4 months 17 days	8 years 6 months 7 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 8.05	$ 10.32
Options Vested, Number	2,210,969	1,369,145
Options Vested, Weighted Average Remaining Contractual Life	7 years 18 days	7 years 5 months 16 days
Weighted average exercise price of options vested (in dollars per share)	$ 11.66	$ 11.60
Weighted average grant date fair value of options granted (in dollars per share)	2.53	5.23	$ 8.93
$0.28 - $2.56
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	0.28
Exercise Price, high end of range (in dollars per share)	$ 2.56
Options Outstanding, Number	191,000
Options Outstanding, Weighted Average Remaining Contractual Life	5 years 7 months 28 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 0.36
Options Vested, Number	184,730
Options Vested, Weighted Average Remaining Contractual Life	5 years 6 months 7 days
Weighted average exercise price of options vested (in dollars per share)	$ 0.29
$3.01 - $4.65
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	3.01
Exercise Price, high end of range (in dollars per share)	$ 4.65
Options Outstanding, Number	2,027,500
Options Outstanding, Weighted Average Remaining Contractual Life	9 years 9 months
Weighted average exercise price of options outstanding (in dollars per share)	$ 3.61
Options Vested, Number	32,375
Options Vested, Weighted Average Remaining Contractual Life	6 years 4 days
Weighted average exercise price of options vested (in dollars per share)	$ 4.64
$4.84 - $7.59
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	4.84
Exercise Price, high end of range (in dollars per share)	$ 7.59
Options Outstanding, Number	1,061,522
Options Outstanding, Weighted Average Remaining Contractual Life	8 years 11 months 9 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 5.95
Options Vested, Number	261,504
Options Vested, Weighted Average Remaining Contractual Life	8 years 10 months 24 days
Weighted average exercise price of options vested (in dollars per share)	$ 5.98
$9.52 - $14.43
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	9.52
Exercise Price, high end of range (in dollars per share)	$ 14.43
Options Outstanding, Number	1,051,738
Options Outstanding, Weighted Average Remaining Contractual Life	7 years 11 months 23 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 10.83
Options Vested, Number	593,596
Options Vested, Weighted Average Remaining Contractual Life	7 years 11 months 5 days
Weighted average exercise price of options vested (in dollars per share)	$ 10.90
$15.00 - $16.70
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	15.00	15.00
Exercise Price, high end of range (in dollars per share)	$ 16.70	$ 16.70
Options Outstanding, Number	1,311,650	1,348,526
Options Outstanding, Weighted Average Remaining Contractual Life	6 years 6 months 4 days	7 years 6 months
Weighted average exercise price of options outstanding (in dollars per share)	$ 15.49	$ 15.49
Options Vested, Number	1,138,764	834,456
Options Vested, Weighted Average Remaining Contractual Life	6 years 5 months 9 days	7 years 4 months 17 days
Weighted average exercise price of options vested (in dollars per share)	$ 15.40	$ 15.37
$0.28 - $4.65
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		0.28
Exercise Price, high end of range (in dollars per share)		$ 4.65
Options Outstanding, Number		301,803
Options Outstanding, Weighted Average Remaining Contractual Life		6 years 5 months 9 days
Weighted average exercise price of options outstanding (in dollars per share)		$ 0.72
Options Vested, Number		261,373
Options Vested, Weighted Average Remaining Contractual Life		6 years 5 months 1 day
Weighted average exercise price of options vested (in dollars per share)		$ 0.68
$5.94 - $9.52
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		5.94
Exercise Price, high end of range (in dollars per share)		$ 9.52
Options Outstanding, Number		1,101,750
Options Outstanding, Weighted Average Remaining Contractual Life		9 years 11 months 1 day
Weighted average exercise price of options outstanding (in dollars per share)		$ 6.10
Options Vested, Number		3,585
Options Vested, Weighted Average Remaining Contractual Life		8 years 8 months 9 days
Weighted average exercise price of options vested (in dollars per share)		$ 8.92
$9.60 - $14.43
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		9.60
Exercise Price, high end of range (in dollars per share)		$ 14.43
Options Outstanding, Number		1,026,180
Options Outstanding, Weighted Average Remaining Contractual Life		8 years 11 months 19 days
Weighted average exercise price of options outstanding (in dollars per share)		$ 10.89
Options Vested, Number		269,731
Options Vested, Weighted Average Remaining Contractual Life		8 years 8 months 12 days
Weighted average exercise price of options vested (in dollars per share)		$ 10.56