SHARE CAPITAL AND CAPITAL IN EXCESS OF PAR VALUE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 733,804
Balance at the end of the period			$ 750,497	$ 733,804
Treasury stock, shares			88	85
Total Number of Shares Purchased			594	921	818
Average Price Paid Per Share (in dollars per share)			$ 32.48	$ 37.01	$ 35.51
Stock dividend rate (as a percent)	3.00%	3.00%	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			$ 26,361	$ 26,181	$ 25,960
Balance at the beginning of the period (in shares)			37,960	37,701	37,382
Issuance of 3% stock dividend			$ 781	$ 781	$ 772
Issuance of 3% stock dividend (in shares)			1,125	1,124	1,111
Conversion of Class B common shares to common shares			$ 37	$ 39	$ 18
Conversion of Class B common shares to common shares (in shares)			53	56	26
Purchase and retirement of common shares			$ (412)	$ (640)	$ (569)
Purchase and retirement of common shares (in shares)			(594)	(921)	(818)
Balance at the end of the period			$ 26,767	$ 26,361	$ 26,181
Balance at the end of the period (in shares)			38,544	37,960	37,701
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)			3	2	3
Balance at the end of the period			$ (1,992)	$ (1,992)	$ (1,992)
Treasury stock, shares			88	85	83	80
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 656,752	$ 646,768	$ 622,882
Issuance of 3% stock dividend			58,688	43,477	52,410
Purchase and retirement of common shares			(18,905)	(33,493)	(28,524)
Balance at the end of the period			696,535	656,752	646,768
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 17,285	$ 16,820	$ 16,348
Balance at the beginning of the period (in shares)			24,891	24,221	23,542
Issuance of 3% stock dividend			$ 519	$ 504	$ 490
Issuance of 3% stock dividend (in shares)			746	726	705
Conversion of Class B common shares to common shares			$ (37)	$ (39)	$ (18)
Conversion of Class B common shares to common shares (in shares)			(53)	(56)	(26)
Balance at the end of the period			$ 17,767	$ 17,285	$ 16,820
Balance at the end of the period (in shares)			25,584	24,891	24,221