Share-based payment plans - Valuation of Share-Based Compensation Awards (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability classified awards
Valuation of Share-Based Compensation Awards
Risk-free interest rate, minimum	1.70%	1.70%	0.70%
Risk-free interest rate, maximum	1.90%	1.90%	1.40%
Dividend yield	1.80%	1.80%	2.20%
Expected lives (in years)	5 years 4 months 24 days
Volatility, minimum	14.00%	14.00%	20.00%
Volatility, maximum	23.00%	25.00%	26.00%
Liability classified awards \| Minimum
Valuation of Share-Based Compensation Awards
Expected lives (in years)	3 months 18 days	1 year	1 year
Liability classified awards \| Maximum
Valuation of Share-Based Compensation Awards
Expected lives (in years)		6 years 6 months	7 years
Equity-settled SARs, RSUs and DSUs
Valuation of Share-Based Compensation Awards
Risk-free interest rate, minimum	1.90%	0.60%	0.60%
Risk-free interest rate, maximum	2.30%	1.90%	1.30%
Volatility, minimum	22.00%	17.00%	17.00%
Volatility, maximum	41.00%	25.00%	25.00%
Equity-settled SARs, RSUs and DSUs \| Minimum
Valuation of Share-Based Compensation Awards
Dividend yield	2.20%	1.50%	1.50%
Expected lives (in years)	3 years	4 months 24 days	4 months 24 days
Equity-settled SARs, RSUs and DSUs \| Maximum
Valuation of Share-Based Compensation Awards
Dividend yield	9.10%	2.20%	2.20%
Expected lives (in years)	7 years	7 years	7 years

No definition available.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

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The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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No definition available.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

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- Definition

The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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No definition available.

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- Definition

The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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No definition available.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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