FAIR VALUE (Carrying Amounts and Fair Values of Company's Financial Instruments) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Financial Liabilities
Securities sold under retail repurchase agreements and federal funds purchased	$ 139,647		$ 119,359
Advances from FHLB	1,063,777		765,833
Subordinated debentures	37,495	$ 37,564	0
Carying amount [member]
Financial Assets
Other equity securities	74,442		45,518
Loans, net of allowance	6,201,580		4,268,991
Other assets	109,498		95,730
Financial Liabilities
Time Deposits	1,245,189		688,951
Securities sold under retail repurchase agreements and federal funds purchased	139,647		119,359
Advances from FHLB	1,063,777		765,833
Subordinated debentures	37,495		0
Estimated fair value [member]
Financial Assets
Other equity securities	74,442		45,518
Loans, net of allowance	6,060,778		4,320,719
Other assets	109,498		95,730
Financial Liabilities
Time Deposits	1,256,431		684,139
Securities sold under retail repurchase agreements and federal funds purchased	139,647		119,359
Advances from FHLB	1,063,894		769,860
Subordinated debentures	31,724		0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets
Other equity securities	0		0
Loans, net of allowance	0		0
Other assets	0		0
Financial Liabilities
Time Deposits	0		0
Securities sold under retail repurchase agreements and federal funds purchased	0		0
Advances from FHLB	0		0
Subordinated debentures	0		0
Significant Other Observable Inputs (Level 2)
Financial Assets
Other equity securities	74,442		45,518
Loans, net of allowance	0		0
Other assets	109,498		95,730
Financial Liabilities
Time Deposits	1,256,431		684,139
Securities sold under retail repurchase agreements and federal funds purchased	139,647		119,359
Advances from FHLB	1,063,894		769,860
Subordinated debentures	0		0
Significant Unobservable Inputs (Level 3)
Financial Assets
Other equity securities	0		0
Loans, net of allowance	6,060,778		4,320,719
Other assets	0		0
Financial Liabilities
Time Deposits	0		0
Securities sold under retail repurchase agreements and federal funds purchased	0		0
Advances from FHLB	0		0
Subordinated debentures	$ 31,724		$ 0