SUMMARY OF QUARTERLY FINANCIAL DATA (UNAUDITED) (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
Consolidated quarterly financial data
Interest income	$ 64,527	$ 68,059	$ 65,664	$ 82,633	$ 62,902	$ 61,090	$ 58,356	$ 73,833	$ 280,883	$ 256,181	$ 218,778
Interest expense	10,132	12,573	11,718	10,334	8,626	8,057	7,272	6,168	44,757	30,123	20,258
NET INTEREST INCOME	54,395	55,486	53,946	72,299	54,276	53,033	51,084	67,665	236,126	226,058	198,520
Provision for loan and lease losses	914	3,153	4,460	17,231	5,104	4,077	4,932	17,255	25,758	31,368	27,704
NET INTEREST INCOME AFTER PROVISION FOR LOAN AND LEASE LOSSES	53,481	52,333	49,486	55,068	49,172	48,956	46,152	50,410	210,368	194,690	170,816
Noninterest income (3)	19,655	12,811	15,125	27,417	10,119	11,465	14,296	27,545	75,008	63,425	58,414
Noninterest expense (4)	40,835	42,411	43,428	45,509	38,963	41,212	40,632	43,045	172,183	163,852	150,844
INCOME BEFORE INCOME TAX EXPENSE	32,301	22,733	21,183	36,976	20,328	19,209	19,816	34,910	113,193	94,263	78,386
Income tax expense (5)	6,533	4,325	3,176	7,460	3,022	1,798	4,150	7,441	21,494	16,411	32,754
NET INCOME	$ 25,768	$ 18,408	$ 18,007	$ 29,516	$ 17,306	$ 17,411	$ 15,666	$ 27,469	$ 91,699	$ 77,852	$ 45,632
Class A Common Stock
Basic earnings per share:
Basic earnings per share (in dollars 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:
Diluted earnings per share (in dollars per share)	1.23	0.88	0.86	1.41	0.83	0.83	0.74	1.32	4.39	3.74	2.20
Dividends declared per common share:
Dividend declared common stock, per share (in dollars per share)	0.264	0.264	0.264	0.264	0.242	0.242	0.242	0.242	1.056	0.968	0.869
Class B Common Stock
Basic earnings per share:
Basic earnings per share (in dollars 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:
Diluted earnings per share (in dollars per share)	1.12	0.80	0.78	1.28	0.75	0.76	0.68	1.20	3.99	3.40	2.00
Dividends declared per common share:
Dividend declared common stock, per share (in dollars per share)	$ 0.240	$ 0.240	$ 0.240	$ 0.240	$ 0.220	$ 0.220	$ 0.220	$ 0.220	$ 0.96	$ 0.88	$ 0.79