Employee plans - Funded Status of the Plan and Amounts Recognized in Consolidated Balance Sheet (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	$ 33.3	$ 45.1	$ 19.9
Other accrued liabilities	(19.3)	(20.7)
Long-term postretirement liabilities	(2,292.6)	(2,111.3)
U.S. Plans
Change in benefit obligation
Benefit obligation at beginning of year	5,231.4	5,665.5
Service cost	0.0	0.0	0.0
Interest cost	231.3	224.1	248.3
Plan participants’ contributions	0.0	0.0
Plan amendment	0.0	(2.7)
Plan curtailment	0.0	0.0
Actuarial loss (gain)	87.2	(285.0)
Benefits paid	(577.9)	(370.5)
Foreign currency translation adjustments	0.0	0.0
Benefit obligation at end of year	4,972.0	5,231.4	5,665.5
Change in plan assets
Fair value of plan assets at beginning of year	3,759.4	4,069.7
Actual return on plan assets	211.8	(5.6)
Employer contribution	58.8	65.8
Plan participants’ contributions	0.0	0.0
Benefits paid	(577.9)	(370.5)
Foreign currency translation adjustments	0.0	0.0
Fair value of plan assets at end of year	3,452.1	3,759.4	4,069.7
Funded status at end of year	(1,519.9)	(1,472.0)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	0.0	0.0
Other accrued liabilities	(6.7)	(6.8)
Long-term postretirement liabilities	(1,513.2)	(1,465.2)
Total funded status	(1,519.9)	(1,472.0)
Accumulated other comprehensive loss, net of tax
Net loss	2,828.8	2,816.2
Prior service (credit) cost	(42.4)	(44.9)
Accumulated benefit obligation	4,972.0	5,231.4
International Plans
Change in benefit obligation
Benefit obligation at beginning of year	2,987.8	3,354.9
Service cost	7.4	8.7	8.4
Interest cost	87.8	94.1	117.9
Plan participants’ contributions	2.3	2.5
Plan amendment	0.0	(32.3)
Plan curtailment	(3.7)	0.0
Actuarial loss (gain)	502.2	(79.5)
Benefits paid	(110.0)	(112.8)
Foreign currency translation adjustments	(397.6)	(247.8)
Benefit obligation at end of year	3,076.2	2,987.8	3,354.9
Change in plan assets
Fair value of plan assets at beginning of year	2,496.8	2,718.9
Actual return on plan assets	287.7	18.6
Employer contribution	73.7	82.5
Plan participants’ contributions	2.3	2.5
Benefits paid	(110.0)	(112.8)
Foreign currency translation adjustments	(320.8)	(212.9)
Fair value of plan assets at end of year	2,429.7	2,496.8	2,718.9
Funded status at end of year	(646.5)	(491.0)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	31.9	43.8
Other accrued liabilities	(0.2)	(0.2)
Long-term postretirement liabilities	(678.2)	(534.6)
Total funded status	(646.5)	(491.0)
Accumulated other comprehensive loss, net of tax
Net loss	1,144.7	1,018.6
Prior service (credit) cost	(27.7)	(35.8)
Accumulated benefit obligation	3,072.1	2,983.1
Other Postretirement Benefit Plans
Change in benefit obligation
Benefit obligation at beginning of year	131.5	150.0
Service cost	0.4	0.6	0.6
Interest cost	6.2	6.9	7.6
Plan participants’ contributions	3.8	4.2
Plan amendment	(3.3)	0.0
Actuarial loss (gain)	(1.4)	(8.0)
Federal drug subsidy	1.4	1.5
Benefits paid	(16.9)	(21.4)
Foreign currency translation adjustments	(1.6)	(2.3)
Benefit obligation at end of year	120.1	131.5	150.0
Change in plan assets
Fair value of plan assets at beginning of year	7.7	9.1
Actual return on plan assets	(0.3)	(0.1)
Employer contribution	13.6	15.9
Plan participants’ contributions	3.8	4.2
Benefits paid	(16.9)	(21.4)
Fair value of plan assets at end of year	7.9	7.7	$ 9.1
Funded status at end of year	(112.2)	(123.8)
Amounts recognized in the consolidated balance sheets consist of:
Prepaid postretirement assets	1.4	1.3
Other accrued liabilities	(12.4)	(13.7)
Long-term postretirement liabilities	(101.2)	(111.4)
Total funded status	(112.2)	(123.8)
Accumulated other comprehensive loss, net of tax
Net loss	19.0	21.3
Prior service (credit) cost	$ (3.2)	$ 0.1