Income taxes - Tax Credit Carryforwards (Details) $ in Millions	Dec. 31, 2019 USD ($)
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	$ 241.8
2020
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	31.5
2021
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	35.0
2022
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	38.1
2023
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	27.0
2024
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	22.5
Thereafter
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	$ 87.7