FAIR VALUES OF FINANCIAL INSTRUMENTS: (Details 4) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Carrying amount and estimated fair value of financial instruments
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 2,974	$ 2,974	$ 2,974	$ 7,132
Securities available-for-sale	1,020,744	926,717
Restricted stock	14,812	15,394
Accrued interest receivable	16,957	18,523
Deposits	(3,755,945)	(3,275,357)
Level 1
Carrying amount and estimated fair value of financial instruments
Cash and due from banks	25,645	26,275
Securities available-for-sale		0
Level 2
Carrying amount and estimated fair value of financial instruments
Cash and due from banks	631,825	101,151
Securities available-for-sale	1,015,713	920,533
Accrued interest receivable	3,521	3,583
Deposits	(3,763,358)	(3,278,099)
Short-term borrowings	(116,061)	(80,119)
Accrued interest payable	(1,033)	(1,739)
Level 3
Carrying amount and estimated fair value of financial instruments
Securities available-for-sale	5,031	6,184
Loans, net	2,560,683	2,648,692
Accrued interest receivable	13,436	14,940
Reported Value Measurement [Member]
Carrying amount and estimated fair value of financial instruments
Cash and due from banks	657,470	127,426
Securities available-for-sale	1,020,744	926,717
Restricted stock	14,812	15,394
Loans, net	2,563,242	2,636,447
Accrued interest receivable	16,957	18,523
Deposits	(3,755,945)	(3,275,357)
Short-term borrowings	(116,061)	(80,119)
Accrued interest payable	(1,033)	(1,739)
Estimate of Fair Value Measurement [Member]
Carrying amount and estimated fair value of financial instruments
Cash and due from banks	657,470	127,426
Securities available-for-sale	1,020,744	926,717
Loans, net	2,560,683	2,648,692
Accrued interest receivable	16,957	18,523
Deposits	(3,763,358)	(3,278,099)
Short-term borrowings	(116,061)	(80,119)
Accrued interest payable	$ (1,033)	$ (1,739)