Income taxes - Deferred tax assets and valuation allowance (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Federal
Operating Loss Carryforwards [Line Items]
Net operating losses	$ 146.4
NOL Gross	7.1
Net operating loss carryforwards that expire	49.0
Net operating losses that never expire	$ 97.4
Net operating loss carryforward taxable income limit percentage	80.00%
State
Operating Loss Carryforwards [Line Items]
NOL carryforward period	12 years
Net operating loss carryforwards that expire	$ 13.9