Employee plans - Components of Net Periodic Benefit Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.0	$ 0.0
Interest cost	231.3	224.1	248.3
Expected return on plan assets	(253.1)	(254.8)	(287.1)
Amortization of prior service cost (credit)	(2.5)	(2.4)	(0.4)
Recognized net actuarial loss	116.0	132.7	109.7
Curtailment gain	0.0	0.0	0.0
Net periodic pension cost	91.7	99.6	70.5
International Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	7.4	8.7	8.4
Interest cost	87.8	94.1	117.9
Expected return on plan assets	(139.5)	(155.4)	(160.5)
Amortization of prior service cost (credit)	(3.0)	(1.9)	(2.1)
Recognized net actuarial loss	40.3	63.6	40.2
Curtailment gain	(2.0)	0.0	(0.6)
Net periodic pension cost	(9.0)	9.1	3.3
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.4	0.6	0.6
Interest cost	6.2	6.9	7.6
Expected return on plan assets	(0.4)	(0.4)	(0.5)
Amortization of prior service cost (credit)	0.0	1.1	1.7
Recognized net actuarial loss	0.5	1.8	1.7
Net periodic pension cost	$ 6.7	$ 10.0	$ 11.1