Stock Based Compensation, Share Option Valuation (Details) - Stock Options [Member] - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Stock Option Activity [Roll Forward]
Outstanding at beginning of period (in shares)	8,615,997
Granted (in shares)	1,397,616
Exercised (in shares)	(106,294)
Cancelled (in shares)	(298,782)
Outstanding at end of period (in shares)	9,608,537		8,615,997
Options vested and exercisable at end of period (in shares)	7,143,825
Options unvested at ending of period (in shares)	2,464,712
Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (in dollars per share)	$ 4.48
Granted (in dollars per share)	14.42
Exercised (in dollars per share)	1.72
Cancelled (in dollars per share)	7.49
Outstanding at end of period (in dollars per share)	5.85		$ 4.48
Options vested and exercisable at end of period (in dollars per share)	2.87
Options unvested at ending (in dollars per share)	$ 14.48
Weighted-Average Remaining Contractual Term [Abstract]
Outstanding	7 years 3 months 18 days		7 years 6 months 4 days
Granted	9 years 6 months 29 days
Options vested and exercisable	6 years 8 months 19 days
Options unvested	9 years 22 days
Aggregate Intrinsic Value [Abstract]
Outstanding at beginning of period	$ 94,474
Exercised	1,110
Outstanding at end of period	88,855		$ 94,474
Options vested and exercisable	84,535
Intrinsic value of options exercised and exercisable	$ 84,500	$ 117,200
Weighted average grant date fair value of shares granted (in dollars per share)	$ 9.50	$ 13.11
Total fair value of options vested	$ 49,400	$ 49,500
Employees and Directors [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.40%	2.61%
Expected term	5 years 9 months 14 days	5 years 9 months 11 days
Expected volatility	75.13%	88.30%
Expected dividend yield	0.00%	0.00%
Exercise price (in dollars per share)	$ 14.42	$ 17.76
Fair value of common stock (in dollars per share)	$ 14.42	$ 17.76