SHARE CAPITAL AND CAPITAL IN EXCESS OF PAR VALUE: (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 691,136
Balance at the beginning of the period (in shares)	78
Balance at the end of the period	$ 698,548	$ 691,136
Balance at the end of the period (in shares)	80	78
Total Number of Shares Purchased	1,047	861	752
Average Price Paid Per Share (in dollars per share)	$ 31.47	$ 29.02	$ 30.73
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	$ 25,892	$ 25,702	$ 25,450
Balance at the beginning of the period (in shares)	37,285	37,011	36,649
Issuance of 3% stock dividend	$ 773	$ 763	$ 761
Issuance of 3% stock dividend (in shares)	1,112	1,099	1,095
Conversion of Class B common shares to common shares	$ 22	$ 25	$ 13
Conversion of Class B common shares to common shares (in shares)	32	36	19
Purchase and retirement of common shares	$ (727)	$ (598)	$ (522)
Purchase and retirement of common shares (in shares)	(1,047)	(861)	(752)
Balance at the end of the period	$ 25,960	$ 25,892	$ 25,702
Balance at the end of the period (in shares)	37,382	37,285	37,011
Class B Common Stock
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 15,894	$ 15,455	$ 15,018
Balance at the beginning of the period (in shares)	22,887	22,256	21,627
Issuance of 3% stock dividend	$ 476	$ 464	$ 450
Issuance of 3% stock dividend (in shares)	687	667	648
Conversion of Class B common shares to common shares	$ (22)	$ (25)	$ (13)
Conversion of Class B common shares to common shares (in shares)	(32)	(36)	(19)
Balance at the end of the period	$ 16,348	$ 15,894	$ 15,455
Balance at the end of the period (in shares)	23,542	22,887	22,256
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)	78	76	73
Issuance of 3% stock dividend (in shares)	2	2	3
Balance at the end of the period	$ (1,992)	$ (1,992)	$ (1,992)
Balance at the end of the period (in shares)	80	78	76
Capital in Excess of Par Value
Changes in share capital and capital in excess of par value
Balance at the beginning of the period	$ 599,186	$ 572,669	$ 547,576
Issuance of 3% stock dividend	55,982	50,939	47,714
Purchase and retirement of common shares	(32,286)	(24,422)	(22,621)
Balance at the end of the period	$ 622,882	$ 599,186	$ 572,669