Employee plans - Schedule of Plan Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plans | U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	$ 3,334.2	$ 3,112.8	$ 3,578.4
Pension Plans | U.S. Plans | Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2,931.5	2,822.9
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	955.3	911.7
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	578.8	494.8
Pension Plans | U.S. Plans | Level 1, 2 and 3 | U.S. Govt. Securities
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	436.0	498.5
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	278.1	374.6
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	433.6	196.6
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	14.0	17.0
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Real Estate, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	186.5	156.7
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(103.5)	35.8
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	135.5	143.7
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Cumulative futures contracts variation margin paid to (received from) brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	8.2	(29.3)
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.0	3.7
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	14.4	20.5
Pension Plans | U.S. Plans | Level 1, 2 and 3 | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(7.4)	(1.4)
Pension Plans | U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,411.8	1,451.6
Pension Plans | U.S. Plans | Level 1 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	952.8	909.0
Pension Plans | U.S. Plans | Level 1 | U.S. Govt. Securities
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	436.0	498.5
Pension Plans | U.S. Plans | Level 1 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	14.0	17.0
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	(8.2)	33.6
Pension Plans | U.S. Plans | Level 1 | Cumulative futures contracts variation margin paid to (received from) brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	8.2	(29.3)
Pension Plans | U.S. Plans | Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.0	3.7
Pension Plans | U.S. Plans | Level 1 | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	14.4	20.5
Pension Plans | U.S. Plans | Level 1 | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(7.4)	(1.4)
Pension Plans | U.S. Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,519.7	1,371.3
Pension Plans | U.S. Plans | Level 2 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2.5	2.7
Pension Plans | U.S. Plans | Level 2 | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	578.8	494.8
Pension Plans | U.S. Plans | Level 2 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	278.1	374.6
Pension Plans | U.S. Plans | Level 2 | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	433.6	196.6
Pension Plans | U.S. Plans | Level 2 | Real Estate, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	186.5	156.7
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	(95.3)	2.2
Pension Plans | U.S. Plans | Level 2 | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	135.5	143.7
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
Pension Plans | U.S. Plans | Plan assets measured using NAV as a practical expedient
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	402.7	289.9
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	86.3	0.0
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	127.0	110.2
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	189.0	179.1
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	0.4	0.6
Pension Plans | International Plans
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	2,816.4	2,539.4	2,833.9
Pension Plans | International Plans | Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,412.3	1,229.8
Pension Plans | International Plans | Level 1, 2 and 3 | Common Stocks
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.0	0.0
Pension Plans | International Plans | Level 1, 2 and 3 | Equity Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	176.7	165.6
Pension Plans | International Plans | Level 1, 2 and 3 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	91.0	145.5
Pension Plans | International Plans | Level 1, 2 and 3 | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.1	123.7
Pension Plans | International Plans | Level 1, 2 and 3 | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	441.0	321.4
Pension Plans | International Plans | Level 1, 2 and 3 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1.0	1.3
Pension Plans | International Plans | Level 1, 2 and 3 | Derivatives, excluding cumulative futures contracts variation margin (received) paid (from) to brokers
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	6.5	2.4
Pension Plans | International Plans | Level 1, 2 and 3 | Other, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	372.8	317.0
Pension Plans | International Plans | Level 1, 2 and 3 | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	189.2	123.6
Pension Plans | International Plans | Level 1, 2 and 3 | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	18.1	29.6
Pension Plans | International Plans | Level 1, 2 and 3 | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.2	2.0
Pension Plans | International Plans | Level 1, 2 and 3 | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(7.3)	(2.3)
Pension Plans | International Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	11.0	29.5
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
Pension Plans | International Plans | Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	18.1	29.6
Pension Plans | International Plans | Level 1 | Receivables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	0.2	2.0
Pension Plans | International Plans | Level 1 | Payables
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	(7.3)	(2.3)
Pension Plans | International Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,278.2	1,076.6
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	176.7	165.6
Pension Plans | International Plans | Level 2 | Other Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	91.0	145.3
Pension Plans | International Plans | Level 2 | Debt Securities, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	441.0	321.4
Pension Plans | International Plans | Level 2 | Real Estate Investment Trusts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1.0	1.3
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	6.5	2.4
Pension Plans | International Plans | Level 2 | Other, Commingled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	372.8	317.0
Pension Plans | International Plans | Level 2 | Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	189.2	123.6
Pension Plans | International Plans | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	123.1	123.7
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	123.1	123.7	135.8
Pension Plans | International Plans | Plan assets measured using NAV as a practical expedient
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	1,404.1	1,309.6
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	406.9	454.9
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	941.0	814.0
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	24.8	23.9
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	31.4	16.8
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	6.9	7.8	7.6
Other Postretirement Plans | U.S. Plans | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	6.9	7.8
Other Postretirement Plans | U.S. Plans | Level 3 | Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Total pension plan assets	$ 6.9	$ 7.8	$ 7.6