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, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 711,538
Balance at the end of the period			$ 733,804	$ 711,538
Treasury stock, shares			85	83
Total Number of Shares Purchased			921	818	1,047
Average Price Paid Per Share (in dollars per share)			$ 37.01	$ 35.51	$ 31.47
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			$ 26,181	$ 25,960	$ 25,892
Balance at the beginning of the period (in shares)			37,701	37,382	37,285
Issuance of 3% stock dividend			$ 781	$ 772	$ 773
Issuance of 3% stock dividend (in shares)			1,124	1,111	1,112
Conversion of Class B common shares to common shares			$ 39	$ 18	$ 22
Conversion of Class B common shares to common shares (in shares)			56	26	32
Purchase and retirement of common shares			$ (640)	$ (569)	$ (727)
Purchase and retirement of common shares (in shares)			(921)	(818)	(1,047)
Balance at the end of the period			$ 26,361	$ 26,181	$ 25,960
Balance at the end of the period (in shares)			37,960	37,701	37,382
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			85	83	80	78
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 646,768	$ 622,882	$ 599,186
Issuance of 3% stock dividend			43,477	52,410	55,982
Purchase and retirement of common shares			(33,493)	(28,524)	(32,286)
Balance at the end of the period			656,752	646,768	622,882
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period			$ 16,820	$ 16,348	$ 15,894
Balance at the beginning of the period (in shares)			24,221	23,542	22,887
Issuance of 3% stock dividend			$ 504	$ 490	$ 476
Issuance of 3% stock dividend (in shares)			726	705	687
Conversion of Class B common shares to common shares			$ (39)	$ (18)	$ (22)
Conversion of Class B common shares to common shares (in shares)			(56)	(26)	(32)
Balance at the end of the period			$ 17,285	$ 16,820	$ 16,348
Balance at the end of the period (in shares)			24,891	24,221	23,542