EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
EARNINGS PER SHARE
Net income	$ 15,804	$ 18,007	$ 42,501	$ 47,523
Dividends declared on Common Stock:
Undistributed net income for basic earnings per share	9,885	12,545	30,652	36,597
Weighted average potential dividends on Class A shares upon exercise of dilutive options	(7)	(32)	(24)	(68)
Undistributed net income for diluted earnings per share	$ 9,878	$ 12,513	$ 30,628	$ 36,529
Weighted average shares outstanding:
Effect of dilutive securities on Class A Shares outstanding	25,000	122,000	42,000	128,000
Weighted average shares outstanding including dilutive securities	21,029,000	21,138,000	21,065,000	21,125,000
Stock option
Diluted earnings per share:
Antidilutive stock options (in shares)	427,000	160,000	247,000	165,000
Stock option | Weighted Average
Diluted earnings per share:
Antidilutive stock options (in shares)	365,000	156,000	172,000	159,000
Class A Common Stock
EARNINGS PER SHARE
Cash dividend premium per share (as a percent)			10.00%
Dividends declared on Common Stock:
Dividends declared on Common Stock	$ (5,347)	$ (4,932)	$ (10,705)	$ (9,865)
Weighted average shares outstanding:
Weighted average shares outstanding	18,804,000	18,804,000	18,821,000	18,785,000
Basic earnings per share:
Per share dividends distributed	$ 0.29	$ 0.26	$ 0.57	$ 0.53
Undistributed earnings per share	0.48	0.60	1.47	1.76
Total basic earnings per share	0.77	0.86	2.04	2.29
Diluted earnings per share:
Per share dividends distributed	0.29	0.26	0.57	0.53
Undistributed earnings per share	0.47	0.60	1.47	1.75
Total diluted earnings per share	$ 0.76	$ 0.86	$ 2.04	$ 2.28
Class B Common Stock
Dividends declared on Common Stock:
Dividends declared on Common Stock	$ (572)	$ (530)	$ (1,144)	$ (1,061)
Weighted average shares outstanding:
Weighted average shares outstanding	2,200,000	2,212,000	2,202,000	2,212,000
Basic earnings per share:
Per share dividends distributed	$ 0.26	$ 0.24	$ 0.52	$ 0.48
Undistributed earnings per share	0.43	0.55	1.34	1.60
Total basic earnings per share	0.69	0.79	1.86	2.08
Diluted earnings per share:
Per share dividends distributed	0.26	0.24	0.52	0.48
Undistributed earnings per share	0.43	0.54	1.33	1.59
Total diluted earnings per share	$ 0.69	$ 0.78	$ 1.85	$ 2.07