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Performance share awards (Details 4) - Performance shares [Member] - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share-based compensation arrangement by share-based payment award [Line Items] |  |  |
Weighted average grant-date fair value | $ 36.19 |  |
Fair value of vested shares | $ 9,700,000 | $ 9,700,000 | $ 0
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward] |  |  |
Nonvested at beginning of period | 573,503 |  |
Nonvested at beginning of period | $ 30.81 |  |
Granted | 285,980 |  |
Weighted average grant-date fair value | $ 36.19 |  |
Additional performance shares earned | 123,147 |  |
Additional performance shares earned | $ 27.48 |  |
Vested | 369,456 |  |
Vested | $ 29.84 |  |
Nonvested at end of period | 613,174 | 573,503 |
Nonvested at end of period | $ 33.24 | $ 30.81 |
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 2020 [Member] |  |  |
Share-based compensation arrangement by share-based payment award [Line Items] |  |  |
Target grant of shares under performance awards | 285,980 |  |
Market Condition [Member] |  |  |
Share-based compensation arrangement by share-based payment award [Line Items] |  |  |
Historical volatility rate | 50.00% |  |
Implied volatility rate | 50.00% |  |
Weighted average grant-date fair value | $ 40.75 | $ 35.07 | $ 34.55
Expected volatility rate, minimum | 15.30% | 19.50% | 17.87%
Expected volatility rate, maximum | 15.97% | 19.69% | 22.14%
Risk-free interest rate, minimum | 1.45% | 2.46% | 1.86%
Risk-free interest rate, maximum | 1.62% | 2.55% | 2.46%
Weighted average discounted dividends per share | $ 2.91 | $ 2.85 | $ 2.46
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward] |  |  |
Weighted average grant-date fair value | $ 40.75 | $ 35.07 | $ 34.55
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] |  |  |
Weighted average grant-date fair value | $ 31.63 | 26.25 | 27.48
Share-based compensation arrangement by share-based payment award, options, outstanding [Roll Forward] |  |  |
Weighted average grant-date fair value | $ 31.63 | $ 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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- Definition 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). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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 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)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + 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 (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options. + 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 (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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 (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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. + 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)(iii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + References No definition available. + Details
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- Definition 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. + Details
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- Definition The maximum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition Number of shares authorized for issuance under share-based payment arrangement. + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + 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 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + References No definition available. + Details
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