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Accounting for Stock-based Compensation - Outstanding Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands | 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] |  |  |
Number of Shares, Outstanding Ending Balance | 38,227 |  |
Number of Shares, Exercisable at December 31, 2020 | 38,227 |  |
Weighted Average Exercise Price, Exercisable at December 31, 2020 | $ 31.93 |  |
Employee Stock Option |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Number of Shares, Outstanding Beginning Balance | 108,128 | 201,810 | 411,498
Number of Shares, Exercised | (69,901) | (93,682) | (209,688)
Number of Shares, Granted | 0 | 0 | 0
Number of Shares, Outstanding Ending Balance | 38,227 | 108,128 | 201,810
Number of Shares, Vested plus expected to vest at December 31, 2020 | 38,227 |  |
Number of Shares, Exercisable at December 31, 2020 | 38,227 |  |
Weighted Average Exercise Price, Outstanding Beginning Balance | $ 35.82 | $ 33.68 | $ 30.71
Weighted Average Exercise Price, Exercised | 37.94 | 31.21 | 27.86
Weighted Average Exercise Price, Outstanding Ending Balance | 31.93 | $ 35.82 | $ 33.68
Weighted Average Exercise Price, Vested plus expected to vest at December 31, 2020 | 31.93 |  |
Weighted Average Exercise Price, Exercisable at December 31, 2020 | $ 31.93 |  |
Aggregate Intrinsic Value, Outstanding at December 31,2020 | $ 1,620,688 |  |
Aggregate Intrinsic Value, Vested plus expected to vest at December 31, 2020 | 1,620,688 |  |
Aggregate Intrinsic Value, Exercisable at December 31, 2020 | $ 1,620,688 |  |

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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option 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)(1)(iii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option 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)(1)(iii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding. + 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 Number of options outstanding, including both vested and non-vested 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 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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