Income taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Mar. 31, 2017	Dec. 31, 2015
Income Taxes [Line Items]
Deferred tax assets	$ 0		$ 0
Deferred tax assets, valuation allowance	28,127,611		12,688,401
Increase (decrease) in valuation allowance	15,400,000
U.S. Federal
Income Taxes [Line Items]
Net operating loss carryforward	61,000,000
Research and development tax credit carryforwards	1,200,000
U.S. State
Income Taxes [Line Items]
Net operating loss carryforward	58,500,000
Operating loss carryforward tax affected	4,500,000
Research and development tax credit carryforwards			$ 340,000
U.S. Federal and State
Income Taxes [Line Items]
Operating loss carryforwards that will increase additional paid in capital if used to reduce income taxes payable	$ 120,000
Scenario, Forecast | Accounting Standards Update 2016-09
Income Taxes [Line Items]
Deferred tax assets		$ 120,000
Deferred tax assets, valuation allowance		$ 120,000