Income taxes - Tax Credit Carryforwards (Details) $ in Millions	Dec. 31, 2018 USD ($)
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	$ 251.4
2019
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	11.8
2020
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	24.5
2021
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	41.9
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
Thereafter
Tax Credit Carryforward [Line Items]
Tax credit carryforwards, set to expire	$ 108.1