Commitments and contingencies - Changes in Total Environmental Liabilities (Detail) $ in Millions	6 Months Ended
Jun. 30, 2020 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Environmental liabilities at beginning of period	$ 78.7
Revised obligation estimates	11.7
Environmental payments	(7.4)
Environmental liabilities at end of period	$ 83.0

Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Amount of expense (reversal of expense) which increases (decreases) the accrual for environmental loss contingencies from revisions in estimates to existing obligations. Excludes changes related to business acquisitions, divestitures, or currency movement.

+ References

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Amount of cash outflow to settle environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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