Employee plans - Schedule of Plan Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plans | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	$ 3,112.8	$ 3,578.4	$ 3,452.1
Pension Plans | U.S. Plans | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	911.7	1,465.1
Pension Plans | U.S. Plans | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	494.8	584.4
Pension Plans | U.S. Plans | U.S. Govt. Securities
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	498.5	139.8
Pension Plans | U.S. Plans | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	374.6	830.6
Pension Plans | U.S. Plans | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	196.6
Pension Plans | U.S. Plans | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	17.0	113.5
Pension Plans | U.S. Plans | Real Estate Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	156.7
Pension Plans | U.S. Plans | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		2.6
Pension Plans | U.S. Plans | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	35.8
Pension Plans | U.S. Plans | Cumulative futures contracts variation margin received from brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(29.3)
Pension Plans | U.S. Plans | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	143.7	228.0
Pension Plans | U.S. Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	3.7	34.8
Pension Plans | U.S. Plans | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	20.5	58.1
Pension Plans | U.S. Plans | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(1.4)	(116.7)
Pension Plans | U.S. Plans | Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2,822.9	3,340.2
Pension Plans | U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,451.6	1,704.5
Pension Plans | U.S. Plans | Level 1 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	909.0	1,461.9
Pension Plans | U.S. Plans | Level 1 | U.S. Govt. Securities
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	498.5	139.8
Pension Plans | U.S. Plans | Level 1 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	17.0	113.5
Pension Plans | U.S. Plans | Level 1 | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		13.1
Pension Plans | U.S. Plans | Level 1 | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	33.6
Pension Plans | U.S. Plans | Level 1 | Cumulative futures contracts variation margin received from brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(29.3)
Pension Plans | U.S. Plans | Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	3.7	34.8
Pension Plans | U.S. Plans | Level 1 | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	20.5	58.1
Pension Plans | U.S. Plans | Level 1 | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(1.4)	(116.7)
Pension Plans | U.S. Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,371.3	1,635.7
Pension Plans | U.S. Plans | Level 2 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.7	3.2
Pension Plans | U.S. Plans | Level 2 | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	494.8	584.4
Pension Plans | U.S. Plans | Level 2 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	374.6	830.6
Pension Plans | U.S. Plans | Level 2 | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	196.6
Pension Plans | U.S. Plans | Level 2 | Real Estate Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	156.7
Pension Plans | U.S. Plans | Level 2 | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		(10.5)
Pension Plans | U.S. Plans | Level 2 | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.2
Pension Plans | U.S. Plans | Level 2 | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	143.7	228.0
Pension Plans | U.S. Plans | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | U.S. Plans | Level 3 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	289.9	238.2
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	19.8
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient | Other, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	110.2	105.1
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient | Private Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	179.1	112.4
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.6	0.9
Pension Plans | International Plans
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2,539.4	2,833.9	2,429.7
Pension Plans | International Plans | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | International Plans | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	165.6	84.0
Pension Plans | International Plans | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	145.5	254.1
Pension Plans | International Plans | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.7	135.8
Pension Plans | International Plans | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	321.4	267.5
Pension Plans | International Plans | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1.3	1.4
Pension Plans | International Plans | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		1.2
Pension Plans | International Plans | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.4
Pension Plans | International Plans | Other, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	317.0	357.1
Pension Plans | International Plans | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.6	29.2
Pension Plans | International Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	29.6	25.1
Pension Plans | International Plans | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.0	19.9
Pension Plans | International Plans | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(2.3)	(1.5)
Pension Plans | International Plans | Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,229.8	1,173.8
Pension Plans | International Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	29.5	46.5
Pension Plans | International Plans | Level 1 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | International Plans | Level 1 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.2	2.6
Pension Plans | International Plans | Level 1 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.4
Pension Plans | International Plans | Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	29.6	25.1
Pension Plans | International Plans | Level 1 | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.0	19.9
Pension Plans | International Plans | Level 1 | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(2.3)	(1.5)
Pension Plans | International Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,076.6	991.5
Pension Plans | International Plans | Level 2 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | International Plans | Level 2 | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	165.6	84.0
Pension Plans | International Plans | Level 2 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	145.3	251.5
Pension Plans | International Plans | Level 2 | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	321.4	267.5
Pension Plans | International Plans | Level 2 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1.3	1.0
Pension Plans | International Plans | Level 2 | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		1.2
Pension Plans | International Plans | Level 2 | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.4
Pension Plans | International Plans | Level 2 | Other, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	317.0	357.1
Pension Plans | International Plans | Level 2 | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.6	29.2
Pension Plans | International Plans | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.7	135.8
Pension Plans | International Plans | Level 3 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | International Plans | Level 3 | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.7	135.8	116.2
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,309.6	1,660.1
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	454.9	780.7
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	814.0	837.7
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient | Other, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	23.9	25.2
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient | Private Real Estate
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	16.8	16.5
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient | Private Equity
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Other Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	7.8	7.6	7.9
Other Postretirement Plans | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		7.6
Other Postretirement Plans | Level 3 | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets		7.6
Other Postretirement Plans | U.S. Plans | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	7.8
Other Postretirement Plans | U.S. Plans | Level 3 | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	$ 7.8	$ 7.6	$ 7.9