EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
EARNINGS PER SHARE
Net income	$ 25,768	$ 18,408	$ 18,007	$ 29,516	$ 17,306	$ 17,411	$ 15,666	$ 27,469	$ 91,699	$ 77,852	$ 45,632
Dividends declared on Common Stock:
Undistributed net income for basic earnings per share									69,807	57,821	27,701
Weighted average potential dividends on Class A shares upon exercise of dilutive options									(118)	(102)	(75)
Undistributed net income for diluted earnings per share									$ 69,689	$ 57,719	$ 27,626
Weighted average shares outstanding:
Effect of dilutive securities on Class A Shares outstanding									112,000	105,000	86,000
Weighted average shares outstanding including dilutive securities									21,135,000	21,065,000	21,007,000
Stock option
Diluted earnings per share:
Antidilutive stock options (in shares)									154,750	165,000	4,500
Stock option | Weighted Average
Diluted earnings per share:
Antidilutive stock options (in shares)									151,260	47,712	2,375
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									$ (19,771)	$ (18,076)	$ (16,158)
Weighted average shares outstanding:
Weighted average shares outstanding									18,813,000	18,736,000	18,678,000
Basic earnings per share:
Per share dividends distributed									$ 1.06	$ 0.97	$ 0.87
Undistributed earnings per share									3.35	2.79	1.34
Total basic earnings per share	$ 1.23	$ 0.88	$ 0.86	$ 1.42	$ 0.83	$ 0.84	$ 0.75	$ 1.32	4.41	3.76	2.21
Diluted earnings per share:
Per share dividends distributed									1.06	0.97	0.87
Undistributed earnings per share									3.33	2.77	1.33
Total diluted earnings per share	1.23	0.88	0.86	1.41	0.83	0.83	0.74	1.32	$ 4.39	$ 3.74	$ 2.20
Class B Common Stock
Dividends declared on Common Stock:
Dividends declared on Common Stock									$ (2,121)	$ (1,955)	$ (1,773)
Weighted average shares outstanding:
Weighted average shares outstanding									2,210,000	2,224,000	2,243,000
Basic earnings per share:
Per share dividends distributed									$ 0.96	$ 0.88	$ 0.79
Undistributed earnings per share									3.05	2.53	1.22
Total basic earnings per share	1.13	0.80	0.79	1.29	0.76	0.76	0.68	1.21	4.01	3.41	2.01
Diluted earnings per share:
Per share dividends distributed									0.96	0.88	0.79
Undistributed earnings per share									3.03	2.52	1.21
Total diluted earnings per share	$ 1.12	$ 0.80	$ 0.78	$ 1.28	$ 0.75	$ 0.76	$ 0.68	$ 1.20	$ 3.99	$ 3.40	$ 2.00