INVESTMENTS (Narrative) (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Investments [Line Items]
Net unrealized gains (losses) on available-for-sale securities, net of tax	$ (26,221,000)		$ (26,221,000)		$ (10,300,000)
Deferred tax assets, unrealized losses on available for sale securities	$ 7,200,000		7,200,000		$ 2,800,000
Payments to acquire investments			$ 148,800,000	$ 141,600,000
AFS securities in unrealized loss positions, number of positions | security	354		354		209
AFS and HTM securities in Continuous Unrealized Loss Position	$ 840,200,000		$ 840,200,000		$ 741,200,000
AFS and HTM Securities Unrealized Loss Accumulated in Investments	36,300,000		36,300,000		14,500,000
Fair Value - 12 Months of More	$ 509,704,000		$ 509,704,000		$ 398,360,000
Unrealized Loss as a Percent of Fair Value	4.30%		4.30%		2.00%
Investment securities sold, carrying amount	$ 23,100,000	$ 19,400,000	$ 33,000,000	19,400,000
Gain (Loss) on Sale of Investments			695,000	827,000
Net gain on sale of securities	664,000	$ 827,000	695,000	$ 827,000
Security pledged as collateral, amortized cost	696,600,000		696,600,000		$ 702,500,000
Security pledged as collateral, fair value	666,200,000		666,200,000		691,200,000
Unrealized loss, change in fair value of bank stock equity securities	11,000		(13,000)
Gain on sale of other investment			195,000
Mortgage Backed Securities and Collateralized Mortgage Obligations
Schedule of Investments [Line Items]
AFS and HTM Securities Unrealized Loss Accumulated in Investments	29,100,000		29,100,000		11,200,000
Fair Value - 12 Months of More	509,700,000		509,700,000		398,400,000
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Schedule of Investments [Line Items]
Fair Value - 12 Months of More	328,180,000		328,180,000		233,971,000
Collateralized Mortgage Obligations
Schedule of Investments [Line Items]
Fair Value - 12 Months of More	181,524,000		181,524,000		$ 164,389,000
Debt Securities
Schedule of Investments [Line Items]
Net gain on sale of securities	(275,000)		(244,000)
Visa Inc. Class B Common Stock
Schedule of Investments [Line Items]
Investment securities sold, carrying amount	441,000		441,000
Gain on sale of other investment	$ 939,000		$ 939,000