INCENTIVE STOCK PLANS - Summary of Performance Shares (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares reserved for performance shares granted during year (in shares)	232,684	213,154	226,448
Weighted average fair value of performance share units granted (in dollars per share)	$ 35.99	$ 40.27	$ 32.17
Intrinsic value of outstanding performance share units	$ 10,758	$ 9,229	$ 10,414
Fair value of performance shares vested	6,387	5,670	0
Cash used to purchase common shares from current and former employees to pay minimum withholding tax requirements on restricted shares vested	$ 2,639	$ 2,651	$ 0

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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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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