Income taxes, Components of the provisions for income taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal corporate tax rate	21.00%	35.00%
Provisional tax benefit	$ (17.9)	$ (13.4)
Provisional tax benefit, adjustment	4.5
Estimated undistributed foreign E&P		95.4
Provisional tax expense	13.4	$ 13.6
Provisional transition tax amount reduction	0.2
U.S. federal cash taxes on deemed mandatory repatriation period		8 years
Current [Abstract]
Federal	1.8	$ 29.4	$ 29.2
State	2.4	3.0	2.3
International	6.0	0.3	1.0
Total current	10.2	32.7	32.5
Deferred [Abstract]
Federal	7.5	(6.0)	10.0
State	3.0	(0.6)	(0.2)
International	(1.9)	9.9	(5.6)
Total deferred	8.6	3.3	4.2
Total provision for income taxes	$ 18.8	$ 36.0	$ 36.7