Income taxes - Tax Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. Federal	$ 348.2
State and local	247.8
Foreign	245.1
Total tax loss carryforwards	841.1	$ 860.0
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	841.1
2020
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	23.9
2021
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	13.5
2022
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	15.8
2023
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	13.3
2024
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	12.2
Thereafter
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	$ 762.4