Summary of significant accounting policies - Recent accounting pronouncements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
consolidated statements of operations
Net product sales	$ 15,393	$ 12,503	$ 11,363	$ 10,729	$ 9,222	$ 7,100	$ 5,179	$ 5,631	$ 49,988	$ 27,132	$ 10,033
Net product sales	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Cost of goods sold									$ 26,928	$ 14,030	$ 11,734
Cost of goods sold									us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Gross profit	$ 7,631	$ 5,546	$ 4,376	$ 5,508	$ 5,582	$ 4,277	$ 2,362	$ 881	$ 23,060	$ 13,102	$ (1,701)
Net loss attributable to common stock	$ (9,337)	$ (12,695)	$ (15,207)	$ (14,436)	$ (13,652)	$ (16,254)	$ (18,645)	$ (17,221)
Net loss per share of common stock, basic and diluted	$ (0.23)	$ (0.43)	$ (0.52)	$ (0.50)	$ (0.47)	$ (0.58)	$ (0.82)	$ (0.88)	$ (1.60)	$ (2.66)	$ (5.16)
Condensed consolidated balance sheet
Inventories	$ 10,367				$ 11,732				$ 10,367	$ 11,732
Other current assets	4,032				3,575				4,032	3,575
Total current assets	88,678				79,395				88,678	79,395
Accrued expenses	35,818				20,944				35,818	20,944
Total current liabilities	57,105				33,300				57,105	33,300
Accumulated deficit	(316,983)				(265,308)				(316,983)	(265,308)
Total liabilities and stockholders' equity	$ 111,357				104,108				$ 111,357	104,108
ASU 2016-02 | Adjustment | Minimum
Leases
Recognition of additional operating lease assets due to adoption of New Lease Standard												$ 3,000
Recognition of additional operating lease liabilities due to adoption of New Lease Standard												3,000
ASU 2016-02 | Adjustment | Maximum
Leases
Recognition of additional operating lease assets due to adoption of New Lease Standard												4,800
Recognition of additional operating lease liabilities due to adoption of New Lease Standard												$ 4,800
As Reported
consolidated statements of operations
Net product sales										25,018	$ 9,154
Cost of goods sold										12,391	11,437
Gross profit										12,627	(2,283)
Net loss attributable to common stock										$ (66,247)	$ (83,333)
Net loss per share of common stock, basic and diluted										$ (2.68)	$ (5.19)
Condensed consolidated balance sheet
Inventories					13,459					$ 13,459
Other current assets					5,093					5,093
Total current assets					82,640					82,640
Accrued expenses					10,570					10,570
Deferred revenue					14,676					14,676
Total current liabilities					37,602					37,602
Accumulated deficit					(266,365)					(266,365)
Total liabilities and stockholders' equity					107,353					107,353
New Revenue Standard Adjustment | ASU 2014-09
consolidated statements of operations
Net product sales										2,114	$ 879
Cost of goods sold										1,639	297
Gross profit										475	582
Net loss attributable to common stock										$ 475	$ 582
Net loss per share of common stock, basic and diluted										$ 0.02	$ 0.03
Condensed consolidated balance sheet
Inventories					(1,727)					$ (1,727)
Other current assets					(1,518)					(1,518)
Total current assets					(3,245)					(3,245)
Accrued expenses					10,374					10,374
Deferred revenue					(14,676)					(14,676)
Total current liabilities					(4,302)					(4,302)
Accumulated deficit					1,057					1,057
Total liabilities and stockholders' equity					(3,245)					(3,245)
As Adjusted | ASU 2014-09
consolidated statements of operations
Net product sales										27,132	$ 10,033
Cost of goods sold										14,030	11,734
Gross profit										13,102	(1,701)
Net loss attributable to common stock										$ (65,772)	$ (82,751)
Net loss per share of common stock, basic and diluted										$ (2.66)	$ (5.16)
Condensed consolidated balance sheet
Inventories					11,732					$ 11,732
Other current assets					3,575					3,575
Total current assets					79,395					79,395
Accrued expenses					20,944					20,944
Total current liabilities					33,300					33,300
Accumulated deficit					(265,308)					(265,308)
Total liabilities and stockholders' equity					$ 104,108					$ 104,108