Income taxes - Operating Loss Carryforwards (Details) $ in Millions	Dec. 31, 2018 USD ($)
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 27.9
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	27.9
2020-2025 | State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	2.8
2026-2031 | State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	6.7
2032-2037 | State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 18.4