Stock-based compensation - Schedule of fair value assumptions (Details) - Options to purchase common stock	10 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum (as a percent)	1.59%	1.23%	1.62%
Risk-free interest rate, maximum (as a percent)	2.71%	2.40%	2.49%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility (as a percent)	75.80%	71.01%	72.98%
Expected term (in years)	6 years	5 years 3 months 18 days	5 years 3 months 18 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility (as a percent)	76.40%	74.20%	74.94%
Expected term (in years)	10 years	10 years	10 years