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INCENTIVE STOCK PLANS - Summary of Performance Shares (Details) - Performance Shares - 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] |  |  |
Common shares reserved for performance shares granted during year (in shares) | 361,870 | 232,684 | 213,154
Weighted average fair value of performance share units granted (in dollars per share) | $ 29.59 | $ 35.99 | $ 40.27
Intrinsic value of outstanding performance share units | $ 11,711 | $ 10,758 | $ 9,229
Fair value of performance shares vested | 3,522 | 6,387 | 5,670
Cash used to purchase common shares from current and former employees to pay withholding tax requirements on restricted shares vested | $ 992 | $ 2,639 | $ 2,651

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- Definition Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585 + 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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