Income taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Statutory income tax rate	35.00%	35.00%	35.00%
Interest or penalties recognized relating to unrecognized tax benefits	$ 0	$ 0	$ 0
Federal
Income Taxes [Line Items]
Net operating loss carryforward	$ 77,300,000	$ 31,900,000
Net operating loss carryforward, expiration beginning year	2033	2033
Tax Credits	$ 13,500,000
Tax Credits, expiration beginning year	2033
State | Research Credits
Income Taxes [Line Items]
Tax Credits	$ 1,300,000