Performance share awards (Details 4) - Performance shares [Member] - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based compensation arrangement by share-based payment award [Line Items]
Granted	$ 30.66
Fair value of vested shares	$ 9,700,000	$ 0	$ 9,600,000
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Nonvested at beginning of period	668,791
Nonvested at beginning of period	$ 23.03
Granted	327,194
Granted	$ 30.66
Additional performance shares earned	103,159
Additional performance shares earned	$ 14.60
Vested	398,919
Vested	$ 15.52
Forfeited	126,722
Forfeited	$ 24.31
Nonvested at end of period	573,503	668,791
Nonvested at end of period	$ 30.81	$ 23.03
Grant Date February 2019 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	327,194
Grant date February 2018 [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Target grant of shares under performance awards	246,309
Market Condition [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Historical volatility rate	50.00%
Implied volatility rate	50.00%
Granted	$ 35.07	$ 34.55	$ 24.31
Expected volatility rate, minimum	19.50%	17.87%	22.70%
Expected volatility rate, maximum	19.69%	22.14%	25.56%
Risk-free interest rate, minimum	2.46%	1.86%	0.69%
Risk-free interest rate, maximum	2.55%	2.46%	1.61%
Weighted average discounted dividends per share	$ 2.85	$ 2.46	$ 1.70
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Granted	$ 35.07	$ 34.55	$ 24.31
Market Condition [Member] \| Minimum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	0.00%
Market Condition [Member] \| Maximum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	200.00%
Performance Condition [Member]
Share-based compensation arrangement by share-based payment award [Line Items]
Granted	$ 26.25	27.48
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward]
Granted	$ 26.25	$ 27.48
Performance Condition [Member] \| Minimum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	0.00%
Performance Condition [Member] \| Maximum
Share-based compensation arrangement by share-based payment award [Line Items]
Participant target grant of shares, percentage rate range	200.00%

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Implied Volatility Rate

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Participant Target Grant of Shares, Percentage Rate Range.

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

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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.

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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).

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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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- Definition

The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends.

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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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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.

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

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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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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Other than shares newly issued, the number of additional shares issued (for example, a stock split) or canceled (for example, to correct a share issuance), during the period under the plan.

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Weighted average price at which grantees could acquire or could have acquired the underlying shares with respect to any other type of change in shares reserved for issuance.

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Weighted average grant-date fair value of options vested.

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