Stock Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Stock Based Compensation
Contractual life (in years)			10 years
Vesting period (in years)			4 years
The activity related to stock option grants to employees and nonemployees
Outstanding at beginning (in shares)			1,764,287
Granted (in shares)		629,720	1,190,501	629,720
Exercised (in shares)			(97,225)
Forfeited (in shares)			(269,328)
Outstanding at end (in shares)	2,588,235		2,588,235		1,764,287
Vested and exercisable at end (in shares)	992,969		992,969
Vested and expected to vest at end (in shares)	2,588,235		2,588,235
Weighted average exercise price per share
Outstanding at beginning (in dollars per share)			$ 26.81
Granted (in dollars per share)		$ 10.08	9.82	$ 10.08
Exercised (in dollars per share)			3.71
Forfeited (in dollars per share)			41.50
Outstanding at end (in dollars per share)	$ 18.34		18.34		$ 26.81
Vested and exercisable at end (in dollars per share)	29.61		29.61
Vested and expected to vest at end (in dollars per share)	$ 18.34		$ 18.34
Weighted-average remaining contractual life (years)
Outstanding (in years)			7 years 8 months 12 days		8 years
Vested and exercisable at end (in years)			5 years 2 months 12 days
Vested and expected to vest at end (in years)			7 years 8 months 12 days
Unvested stock options (in shares)	1,595,266		1,595,266
Unrecognized compensation cost	$ 10.4		$ 10.4
Weighted average amortization period			3 years
Aggregate intrinsic value of options outstanding	1.4		$ 1.4
Aggregate intrinsic value of options exercisable	$ 1.3		$ 1.3
Weighted average grant date fair value, options granted	$ 4.99		$ 6.81