SHARE CAPITAL AND CAPITAL IN EXCESS OF PAR VALUE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 750,497
Balance at the end of the period	$ 759,649	$ 750,497
Treasury stock, shares	90	88
Total Number of Shares Purchased	923	594	921
Average Price Paid Per Share (in dollars per share)	$ 36.93	$ 32.48	$ 37.01
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	$ 26,767	$ 26,361	$ 26,181
Balance at the beginning of the period (in shares)	38,544	37,960	37,701
Issuance of 3% stock dividend	$ 798	$ 781	$ 781
Issuance of 3% stock dividend (in shares)	1,150	1,125	1,124
Conversion of Class B common shares to common shares	$ 45	$ 37	$ 39
Conversion of Class B common shares to common shares (in shares)	65	53	56
Purchase and retirement of common shares and other	$ (641)	$ (412)	$ (640)
Purchase and retirement of common shares and other (in shares)	(923)	(594)	(921)
Balance at the end of the period	$ 26,969	$ 26,767	$ 26,361
Balance at the end of the period (in shares)	38,836	38,544	37,960
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)
Issuance of 3% stock dividend (in shares)	2	3	2
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)
Treasury stock, shares	90	88	85	83
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 696,535	$ 656,752	$ 646,768
Issuance of 3% stock dividend	32,999	58,688	43,477
Purchase and retirement of common shares and other	(33,475)	(18,905)	(33,493)
Balance at the end of the period	696,059	696,535	656,752
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 17,767	$ 17,285	$ 16,820
Balance at the beginning of the period (in shares)	25,584	24,891	24,221
Issuance of 3% stock dividend	$ 532	$ 519	$ 504
Issuance of 3% stock dividend (in shares)	768	746	726
Conversion of Class B common shares to common shares	$ (45)	$ (37)	$ (39)
Conversion of Class B common shares to common shares (in shares)	(65)	(53)	(56)
Balance at the end of the period	$ 18,254	$ 17,767	$ 17,285
Balance at the end of the period (in shares)	26,287	25,584	24,891