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Share-based compensation - Stock appreciation rights - Awards, simulation inputs, outputs (Details) - $ / shares | Mar. 04, 2021 | Mar. 04, 2020 | Mar. 07, 2019 | Apr. 04, 2018 | Jan. 15, 2016 | Mar. 06, 2015 | Sep. 01, 2014 | Mar. 12, 2014
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Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Valuation Method | Black-Scholes | Black-Scholes | Black-Scholes | Black-Scholes | Monte Carlo | Monte Carlo | Monte Carlo | Monte Carlo
Stock appreciation rights |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
SARs Awarded (in shares) | 610,691 | 549,020 | 560,000 | 1,719,733 | 205,519 | 37,797 | 5,595 | 22,118
Exercise Price (in dollars per share) | $ 4.28 | $ 5.25 | $ 5.10 | $ 7.40 | $ 9.20 | $ 10.25 | $ 13.91 | $ 13.66
Vesting Period (in years) | 3 years | 3 years | 3 years | 3 years | 3 years | 3 years | 3 years | 3 years
Grant Price (in dollars per share) | $ 4.28 | $ 5.25 | $ 5.10 | $ 7.40 | $ 9.20 | $ 10.25 | $ 13.91 | $ 13.66
Dividend Yield (in hundredths) | 0.00% | 0.00% | 0.00% | 0.00% | 6.63% | 3.90% | 2.88% | 2.93%
Risk-free rate of Return (in hundredths) | 0.66% | 0.73% | 2.43% | 2.51% | 1.79% | 1.90% | 2.20% | 2.06%
Expected Volatility (in hundredths) | 55.39% | 46.42% | 43.65% | 40.59% | 58.09% | 61.38% | 53.60% | 56.31%
Weighted Average Fair Value at grant date (in dollars per share) | $ 1.93 | $ 2.04 | $ 2.00 | $ 2.67 | $ 2.20 | $ 2.98 | $ 4.20 | $ 4.17
Average Expected Exercise Life | 4 years 6 months | 4 years 6 months | 4 years 6 months | 4 years 3 months |  |  |  |
Stock appreciation rights | Maximum [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Average Expected Exercise Life |  |  |  |  | 5 years | 5 years | 5 years | 5 years
Stock appreciation rights | Minimum [Member] |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Average Expected Exercise Life |  |  |  |  | 4 years | 4 years 2 months 12 days | 4 years 6 months | 4 years 7 months 6 days

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- Definition Granted price per share of equity-based compensation awards granted. Excludes stock and unit options. + References No definition available. + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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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 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 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 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 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)(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Description of method used to estimate fair value of award 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 (f)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + 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)(iv) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + References No definition available. + Details
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