INCENTIVE STOCK PLANS - Summary of Performance Shares (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
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

Aggregate number of common shares reserved for future issuance.

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Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement.

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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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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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