Income taxes - Tax Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
U.S. Federal	$ 317.6
State and local	200.1
Foreign	277.5
Total tax loss carryforwards	795.2	$ 841.1
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	795.2
2021
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	15.7
2022
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	17.5
2023
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	13.6
2024
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	11.9
2025
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	13.2
Thereafter
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	$ 723.3