FAIR VALUE (Carrying Amounts and Fair Values of Company's Financial Instruments) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Financial Liabilities
Securities sold under retail repurchase agreements and federal funds purchased	$ 149,323		$ 119,359
Advances from FHLB	1,011,109		765,833
Subordinated debentures	37,530	$ 37,564	0
Reported Value Measurement [Member]
Financial Assets
Other equity securities	63,115		45,518
Loans, net of allowance	6,014,620		4,268,991
Other assets	108,839		95,730
Financial Liabilities
Time Deposits	1,206,603		688,951
Securities sold under retail repurchase agreements and federal funds purchased	149,323		119,359
Advances from FHLB	1,011,109		765,833
Subordinated debentures	37,530		0
Estimate of Fair Value Measurement [Member]
Financial Assets
Other equity securities	63,115		45,518
Loans, net of allowance	6,043,115		4,320,719
Other assets	108,839		95,730
Financial Liabilities
Time Deposits	1,222,469		684,139
Securities sold under retail repurchase agreements and federal funds purchased	149,323		119,359
Advances from FHLB	1,012,398		769,860
Subordinated debentures	37,530		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	63,115		45,518
Loans, net of allowance	0		0
Other assets	108,839		95,730
Financial Liabilities
Time Deposits	1,222,469		684,139
Securities sold under retail repurchase agreements and federal funds purchased	149,323		119,359
Advances from FHLB	1,012,398		769,860
Subordinated debentures	0		0
Significant Unobservable Inputs (Level 3)
Financial Assets
Other equity securities	0		0
Loans, net of allowance	6,043,115		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	$ 37,530		$ 0