Compensation plans - Restricted stock option awards activity (Details) - Stock option awards - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock option awards
Outstanding at the beginning of the period (shares)	11	17	127
Exercised (shares)	(2)		(1)
Expired or canceled (shares)	(2)	(6)	(92)
Forfeited (shares)			(17)
Outstanding at the end of the period (shares)	7	11	17	127
Weighted-average exercise price (per award)
Outstanding at the end of the period (USD per share)	$ 257.42	$ 251.20	$ 253.80
Exercised (USD per share)	82.00		82.00
Expired or canceled (USD per share)	374.77	238.38	271.00
Forfeited (USD per share)			172.20
Outstanding at end of the period (USD per share)	$ 275.88	$ 257.42	$ 251.20	$ 253.80
Weighted-average remaining contractual term (years)
Outstanding at the end of the period	3 years 2 months 26 days	4 years	5 years	5 years 11 months 26 days
Intrinsic value, options exercisable	$ 0

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Number of options outstanding, including both vested and non-vested options.

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-Publisher FASB
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- Definition

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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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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