SHARE CAPITAL AND CAPITAL IN EXCESS OF PAR VALUE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	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	$ 698,548
Balance at the beginning of the period (in shares)	80
Balance at the end of the period	$ 711,538	$ 698,548
Balance at the end of the period (in shares)	83	80
Total Number of Shares Purchased	818	1,047	861
Average Price Paid Per Share (in dollars per share)	$ 35.51	$ 31.47	$ 29.02
Stock dividend rate (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	$ 25,960	$ 25,892	$ 25,702
Balance at the beginning of the period (in shares)	37,382	37,285	37,011
Issuance of 3% stock dividend	$ 772	$ 773	$ 763
Issuance of 3% stock dividend (in shares)	1,111	1,112	1,099
Conversion of Class B common shares to common shares	$ 18	$ 22	$ 25
Conversion of Class B common shares to common shares (in shares)	26	32	36
Purchase and retirement of common shares	$ (569)	$ (727)	$ (598)
Purchase and retirement of common shares (in shares)	(818)	(1,047)	(861)
Balance at the end of the period	$ 26,181	$ 25,960	$ 25,892
Balance at the end of the period (in shares)	37,701	37,382	37,285
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)	80	78	76
Issuance of 3% stock dividend (in shares)	3	2	2
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)
Balance at the end of the period (in shares)	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	$ 622,882	$ 599,186	$ 572,669
Issuance of 3% stock dividend	52,410	55,982	50,939
Purchase and retirement of common shares	(28,524)	(32,286)	(24,422)
Balance at the end of the period	646,768	622,882	599,186
Class B Common Stock | Common Stock.
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 16,348	$ 15,894	$ 15,455
Balance at the beginning of the period (in shares)	23,542	22,887	22,256
Issuance of 3% stock dividend	$ 490	$ 476	$ 464
Issuance of 3% stock dividend (in shares)	705	687	667
Conversion of Class B common shares to common shares	$ (18)	$ (22)	$ (25)
Conversion of Class B common shares to common shares (in shares)	(26)	(32)	(36)
Balance at the end of the period	$ 16,820	$ 16,348	$ 15,894
Balance at the end of the period (in shares)	24,221	23,542	22,887