Share-based compensation plans - Stock activity - RSU, PSU, DSU (Details) - $ / shares	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Restricted Stock Units
Shares (in thousands)
Nonvested, beginning balance, shares	251,800
Granted, shares	110,100
Forfeited, shares	(8,500)
Vested, shares	(77,500)
Nonvested, ending balance, shares	275,900	251,800
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning balance (dollars per share)	$ 72.30
Granted (dollars per share)	78.35
Forfeited (dollars per share)	65.95
Vested (dollars per share)	74.07
Nonvested, ending balance (dollars per share)	$ 74.42	$ 72.30
Performance Shares Units PSU
Shares (in thousands)
Nonvested, beginning balance, shares	576,700
Granted, shares	290,500
Forfeited, shares	(15,700)
Vested, shares	(149,500)
Nonvested, ending balance, shares	702,000.0	576,700
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning balance (dollars per share)	$ 80.43
Granted (dollars per share)	80.59
Forfeited (dollars per share)	79.03
Vested (dollars per share)	74.03
Nonvested, ending balance (dollars per share)	$ 81.90	$ 80.43
Actual shares earned and distributed (shares)		246,900
Target shares (shares)		149,500
Deferred Stock Units
Shares (in thousands)
Nonvested, beginning balance, shares	21,600
Granted, shares	17,100
Forfeited, shares	(3,000.0)
Vested, shares	(21,400)
Nonvested, ending balance, shares	14,300	21,600
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning balance (dollars per share)	$ 40.46
Granted (dollars per share)	79.04
Forfeited (dollars per share)	40.46
Vested (dollars per share)	45.68
Nonvested, ending balance (dollars per share)	$ 78.74	$ 40.46

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(3)
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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-Publisher FASB
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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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-Publisher FASB
-Name Accounting Standards Codification
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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

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-Publisher FASB
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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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-Publisher FASB
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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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- Definition

Number of shares authorized for issuance under share-based payment arrangement.

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Number of shares issued under share-based payment arrangement.

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