Quarterly financial information (unaudited) (Details) - USD ($)	3 Months Ended								10 Months Ended		12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net product revenue	$ 291,099	$ 586,134	$ 112,685	$ 0						$ 0	$ 989,918	$ 0
Other revenue	7,805	12,940	0	0						0	20,745	0
Total revenue	298,904	599,074	112,685	0						0	1,010,663	0
Costs and expenses:
Cost of product revenue	133,706	221,090	110,931	197,020						0	662,747	0
Research and development	4,230,804	5,665,357	6,094,921	4,387,079	$ 3,309,055	$ 3,445,200	$ 3,190,178	$ 2,804,946		4,003,911	20,378,161	12,749,379
Selling, general and administrative	3,918,661	5,447,847	5,377,784	5,111,695	4,621,383	4,722,281	3,904,403	3,216,212		4,025,895	19,855,987	16,464,279
Total costs and expenses	8,283,171	11,334,294	11,583,636	9,695,794	7,930,438	8,167,481	7,094,581	6,021,158		8,029,806	40,896,895	29,213,658
Loss from operations	(7,984,267)	(10,735,220)	(11,470,951)	(9,695,794)	(7,930,438)	(8,167,481)	(7,094,581)	(6,021,158)
Interest income, net	84,232	97,726	107,818	103,333	37,631	14,637	16,183	3,577		18,946	393,109	72,028
Net loss	$ (7,900,035)	$ (10,637,494)	$ (11,363,133)	$ (9,592,461)	$ (7,892,807)	$ (8,152,844)	$ (7,078,398)	$ (6,017,581)	$ (8,010,860)	$ (8,010,860)	$ (39,493,123)	$ (29,141,630)
Net loss per share attributable to common stockholders - basic and diluted (in usd per share)	$ (0.48)	$ (0.65)	$ (0.71)	$ (0.61)	$ (0.51)	$ (0.53)	$ (0.47)	$ (0.59)		$ (4.57)	$ (2.43)	$ (2.08)
Weighted-average number of common shares outstanding — basic and diluted	16,619,596	16,361,617	16,105,555	15,843,532	15,551,800	15,290,435	15,034,764	10,179,955		1,753,024	16,233,985	14,032,916