EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
EARNINGS PER SHARE
Net income	$ 15,666	$ 10,071	$ 43,135	$ 30,088
Dividends declared on Common Stock:
Undistributed net income for basic earnings per share	10,661	5,527	33,119	21,226
Weighted average potential dividends on Class A shares upon exercise of dilutive options	(35)	(17)	(64)	(34)
Undistributed net income for diluted earnings per share	$ 10,626	$ 5,510	$ 33,055	$ 21,192
Weighted average shares outstanding:
Effect of dilutive securities on Class A Shares outstanding	144,000	79,000	133,000	78,000
Weighted average shares outstanding including dilutive securities	21,331,000	21,230,000	21,072,000	20,996,000
Stock option
Diluted earnings per share:
Antidilutive stock options (in shares)	3,000	2,500	3,000	2,500
Stock option | Weighted Average
Diluted earnings per share:
Antidilutive stock options (in shares)	400	1,166	200	584
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	$ (4,518)	$ (4,095)	$ (9,035)	$ (7,986)
Weighted average shares outstanding:
Weighted average shares outstanding	18,935,000	18,883,000	18,704,000	18,675,000
Basic earnings per share:
Per share dividends distributed	$ 0.24	$ 0.22	$ 0.48	$ 0.43
Undistributed earnings per share	0.51	0.26	1.60	1.02
Total basic earnings per share	0.75	0.48	2.08	1.45
Diluted earnings per share:
Per share dividends distributed	0.24	0.22	0.48	0.43
Undistributed earnings per share	0.50	0.26	1.58	1.02
Total diluted earnings per share	$ 0.74	$ 0.48	$ 2.06	$ 1.45
Class B Common Stock
Dividends declared on Common Stock:
Dividends declared on Common Stock	$ (487)	$ (449)	$ (981)	$ (876)
Weighted average shares outstanding:
Weighted average shares outstanding	2,252,000	2,268,000	2,235,000	2,243,000
Basic earnings per share:
Per share dividends distributed	$ 0.22	$ 0.20	$ 0.44	$ 0.39
Undistributed earnings per share	0.46	0.24	1.45	0.93
Total basic earnings per share	0.68	0.44	1.89	1.32
Diluted earnings per share:
Per share dividends distributed	0.22	0.20	0.44	0.39
Undistributed earnings per share	0.46	0.24	1.44	0.93
Total diluted earnings per share	$ 0.68	$ 0.44	$ 1.88	$ 1.32