Stock incentive plans, Assumptions used in option valuation calculations (Details) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock Options [Member]
Assumptions used in valuation model [Abstract]
Term of award	10 years
2014 Equity Incentive Plan [Member]
Assumptions used in valuation model [Abstract]
Expected option term	6 years 9 months 18 days	6 years 9 months 18 days	6 years 9 months 18 days
Expected volatility	38.80%	38.00%	36.20%
Risk-free interest rate	2.60%	2.70%	2.10%
Dividend yield	3.50%	2.60%	4.30%
2014 Equity Incentive Plan [Member] \| Stock Options [Member]
Assumptions used in valuation model [Abstract]
Weighted average fair value of grants (in dollars per share)	$ 3.01	$ 4.38	$ 1.95
Term of award	10 years
2014 Equity Incentive Plan [Member] \| Restricted Share Units [Member]
Assumptions used in valuation model [Abstract]
Weighted average fair value of grants (in dollars per share)	$ 10.46	$ 12.91	$ 7.53

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.

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- Definition

The estimated measure of the 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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- Definition

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

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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents 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
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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- Definition

Weighted average grant-date fair value of non-vested options outstanding.

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No definition available.

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