SHARE CAPITAL AND CAPITAL IN EXCESS OF PAR VALUE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 759,649
Balance at the beginning of the period (in shares)	90
Balance at the end of the period	$ 763,100	$ 759,649
Balance at the end of the period (in shares)	93	90
Total Number of Shares Purchased	982	923	594
Average Price Paid Per Share (in dollars per share)	$ 32.59	$ 36.93	$ 32.48
Stock dividends (as a percent)	3.00%	3.00%	3.00%
Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 26,969	$ 26,767	$ 26,361
Balance at the beginning of the period (in shares)	38,836	38,544	37,960
Issuance of 3% stock dividend	$ 804	$ 798	$ 781
Issuance of 3% stock dividend (in shares)	1,157	1,150	1,125
Conversion of Class B common shares to common shares	$ 43	$ 45	$ 37
Conversion of Class B common shares to common shares (in shares)	62	65	53
Purchase and retirement of common shares and other	$ (682)	$ (641)	$ (412)
Purchase and retirement of common shares and other (in shares)	(982)	(923)	(594)
Balance at the end of the period	$ 27,134	$ 26,969	$ 26,767
Balance at the end of the period (in shares)	39,073	38,836	38,544
Treasury Stock
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ (1,992)	$ (1,992)	$ (1,992)
Balance at the beginning of the period (in shares)	90	88	85
Issuance of 3% stock dividend (in shares)	3	2	3
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)
Balance at the end of the period (in shares)	93	90	88
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 696,059	$ 696,535	$ 656,752
Issuance of 3% stock dividend	42,244	32,999	58,688
Purchase and retirement of common shares and other	(31,373)	(33,475)	(18,905)
Balance at the end of the period	706,930	696,059	696,535
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 18,254	$ 17,767	$ 17,285
Balance at the beginning of the period (in shares)	26,287	25,584	24,891
Issuance of 3% stock dividend	$ 547	$ 532	$ 519
Issuance of 3% stock dividend (in shares)	787	768	746
Conversion of Class B common shares to common shares	$ (43)	$ (45)	$ (37)
Conversion of Class B common shares to common shares (in shares)	(62)	(65)	(53)
Balance at the end of the period	$ 18,758	$ 18,254	$ 17,767
Balance at the end of the period (in shares)	27,012	26,287	25,584