FAIR VALUE - Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Investment securities available for sale	$ 1,905,230	$ 1,942,568
Held To Maturity Securities	144,673	144,815
Total gross loans held for investment portfolio	8,996,816	8,858,123	$ 8,695,890
Less: allowance for loan and lease losses	(183,732)	(196,362)	$ (225,856)	$ (231,843)
Loans held for investment, net	8,813,084	8,661,761
Liabilities:
Other borrowings	184,150	184,150
Carrying Reported Amount Fair Value Disclosure [Member]
Assets:
Cash and due from banks and money market investments	589,575	586,203
Investment securities available for sale	1,905,230	1,942,568
Held To Maturity Securities	144,673	144,815
Equity Securities	44,438	44,530
Loans held for sale	33,175	43,186
Total gross loans held for investment portfolio	8,996,816	8,858,123
Less: allowance for loan and lease losses	(183,732)	(196,362)
Loans held for investment, net	8,813,084	8,661,761
Asset Derivative	598	1,018
Liabilities:
Deposits	9,070,834	8,994,714
Securities sold under agreements to repurchase	100,000	150,086
Advances from FHLB	740,000	740,000
Other borrowings	184,150	184,150
Liability Derivatives	588	1,000
Estimate Of Fair Value Fair Value Disclosure [Member]
Assets:
Cash and due from banks and money market investments	589,575	586,203
Investment securities available for sale	1,905,230	1,942,568
Held To Maturity Securities	123,906	125,658
Equity Securities	44,438	44,530
Loans held for sale	33,746	43,831
Loans held for investment, net	8,329,345	8,213,144
Asset Derivative	598	1,018
Liabilities:
Deposits	9,073,967	9,005,679
Securities sold under agreements to repurchase	119,361	169,366
Advances from FHLB	735,445	730,253
Other borrowings	184,091	177,201
Liability Derivatives	588	1,000
Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 1 [Member]
Assets:
Cash and due from banks and money market investments	589,575	586,203
Investment securities available for sale	7,485	7,456
Held To Maturity Securities	0	0
Equity Securities	425	418
Loans held for sale	0	0
Loans held for investment, net	0	0
Asset Derivative	0	0
Liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Advances from FHLB	0	0
Other borrowings	0	0
Liability Derivatives	0	0
Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 2 [Member]
Assets:
Cash and due from banks and money market investments	0	0
Investment securities available for sale	1,881,039	1,917,874
Held To Maturity Securities	0	0
Equity Securities	44,013	44,112
Loans held for sale	26,365	27,720
Loans held for investment, net	0	0
Asset Derivative	598	1,018
Liabilities:
Deposits	9,073,967	9,005,679
Securities sold under agreements to repurchase	119,361	169,366
Advances from FHLB	735,445	730,253
Other borrowings	0	0
Liability Derivatives	588	1,000
Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 3 [Member]
Assets:
Cash and due from banks and money market investments	0	0
Investment securities available for sale	16,706	17,238
Held To Maturity Securities	123,906	125,658
Equity Securities	0	0
Loans held for sale	7,381	16,111
Loans held for investment, net	8,329,345	8,213,144
Asset Derivative	0	0
Liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Advances from FHLB	0	0
Other borrowings	184,091	177,201
Liability Derivatives	$ 0	$ 0