INVESTMENTS (Narrative) (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Investments [Line Items]
Net unrealized gains (losses) on available-for-sale securities, net of tax	$ (20,227)		$ (10,300)
Deferred tax assets, unrealized losses on available for sale securities	5,500		$ 2,800
Payments to acquire investments	$ 50,100	$ 77,300
AFS securities in unrealized loss positions, number of positions | security	328		209
AFS and HTM securities in Continuous Unrealized Loss Position	$ 823,600		$ 741,200
AFS and HTM Securities Unrealized Loss Accumulated in Investments	27,900		14,500
Fair Value - 12 Months of More	$ 388,579		$ 398,360
Unrealized Loss as a Percent of Fair Value	3.00%		2.00%
Investment securities sold, carrying amount	$ 19,400
Federal Home Loan Bank Stock	17,639		$ 18,296
Federal Reserve Bank Stock	5,374		5,374
Security pledged as collateral, amortized cost	684,300		702,500
Security pledged as collateral, fair value	661,900		691,200
Unrealized loss, change in fair value of bank stock equity securities	35
Gain on sale of other investment	195
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value - 12 Months of More	388,600
Collateralized Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value - 12 Months of More	398,400
Mortgage Backed Securities and Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
AFS and HTM Securities Unrealized Loss Accumulated in Investments	18,400		11,200
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Schedule of Investments [Line Items]
Fair Value - 12 Months of More	233,957		233,971
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Fair Value - 12 Months of More	$ 154,622		$ 164,389