Income taxes - Tax Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. Federal	$ 349.6
State and local	257.5
Foreign	252.9
Total tax loss carryforwards	860.0	$ 837.6
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	860.0
2019
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	7.7
2020
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	25.9
2021
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	14.1
2022
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	15.1
2023
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	14.3
Thereafter
Operating Loss Carryforwards [Line Items]
Tax loss carryforwards, set to expire	$ 782.9