FAIR VALUES OF FINANCIAL INSTRUMENTS: (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value measurement
Securities available-for-sale	$ 814,931	$ 853,725
U.S. Government entity mortgage-backed securities
Fair value measurement
Securities available-for-sale	13,695	13,249
Mortgage-backed securities, residential
Fair value measurement
Securities available-for-sale	215,338	261,005
Mortgage-backed securities, commercial
Fair value measurement
Securities available-for-sale	1	4
Collateralized mortgage obligations
Fair value measurement
Securities available-for-sale	339,670	348,176
State and municipal obligations
Fair value measurement
Securities available-for-sale	231,622	218,923
Collateralized debt obligations
Fair value measurement
Securities available-for-sale	14,605	12,368
Level 1
Fair value measurement
Securities available-for-sale	0	0
Level 2
Fair value measurement
Securities available-for-sale	796,646	837,147
Derivative Assets	298	653
Derivative Liabilities	(298)	(653)
Level 2 | U.S. Government entity mortgage-backed securities
Fair value measurement
Securities available-for-sale	13,695	13,249
Level 2 | Mortgage-backed securities, residential
Fair value measurement
Securities available-for-sale	215,338	261,005
Level 2 | Mortgage-backed securities, commercial
Fair value measurement
Securities available-for-sale	1	4
Level 2 | Collateralized mortgage obligations
Fair value measurement
Securities available-for-sale	339,670	348,176
Level 2 | State and municipal obligations
Fair value measurement
Securities available-for-sale	227,942	214,713
Level 3
Fair value measurement
Securities available-for-sale	18,285	16,578
Level 3 | State and municipal obligations
Fair value measurement
Securities available-for-sale	3,680	4,210
Level 3 | Collateralized debt obligations
Fair value measurement
Securities available-for-sale	$ 14,605	$ 12,368