INVESTMENT SECURITIES - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	$ 613,137	$ 398,776
Available-for-sale securities, Less than 12 months, Unrealized Loss	13,601	3,624
Available-for-sale securities, 12 months or longer, Fair Value	234,674	239,678
Available-for-sale securities, 12 months or longer, Unrealized Loss	12,487	8,024
Available-for-sale securities, Total, Fair Value	847,811	638,454
Available-for-sale securities, Total, Unrealized Loss	26,088	11,648
Held-to-maturity, Less than 12 months, Fair Value	92,114	23,404
Held-to-maturity, Less than 12 months, Unrealized Loss	708	59
Held-to-maturity, 12 months or longer, Fair Value	5,372	5,625
Held-to-maturity, 12 months or longer, Unrealized Loss	110	32
Held-to-maturity, Total, Fair Value	97,486	29,029
Held-to-maturity, Total, Unrealized Loss	818	91
CMO
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	186,243	146,726
Available-for-sale securities, Less than 12 months, Unrealized Loss	4,292	1,820
Available-for-sale securities, 12 months or longer, Fair Value	77,903	77,149
Available-for-sale securities, 12 months or longer, Unrealized Loss	3,556	2,321
Available-for-sale securities, Total, Fair Value	264,146	223,875
Available-for-sale securities, Total, Unrealized Loss	7,848	4,141
FNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	339,535	204,921
Available-for-sale securities, Less than 12 months, Unrealized Loss	7,581	1,479
Available-for-sale securities, 12 months or longer, Fair Value	122,243	126,342
Available-for-sale securities, 12 months or longer, Unrealized Loss	7,166	4,693
Available-for-sale securities, Total, Fair Value	461,778	331,263
Available-for-sale securities, Total, Unrealized Loss	14,747	6,172
FHLMC MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	76,822	42,514
Available-for-sale securities, Less than 12 months, Unrealized Loss	1,463	269
Available-for-sale securities, 12 months or longer, Fair Value	20,401	21,405
Available-for-sale securities, 12 months or longer, Unrealized Loss	1,101	655
Available-for-sale securities, Total, Fair Value	97,223	63,919
Available-for-sale securities, Total, Unrealized Loss	2,564	924
GNMA MBS
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	10,537	4,615
Available-for-sale securities, Less than 12 months, Unrealized Loss	265	56
Available-for-sale securities, 12 months or longer, Fair Value	14,127	14,782
Available-for-sale securities, 12 months or longer, Unrealized Loss	664	355
Available-for-sale securities, Total, Fair Value	24,664	19,397
Available-for-sale securities, Total, Unrealized Loss	929	411
Other investments
Schedule of Available-for-sale Securities [Line Items]
Other investments, Less than 12 months, Fair Value	0	0
Other investments, Less than 12 months, Unrealized Loss	0	0
Other investments, 12 months or longer, Fair Value	617	624
Other investments, 12 months or longer, Unrealized Loss	30	20
Other investments, Total, Fair Value	617	624
Other investments, Total, Unrealized Loss	30	20
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity, Less than 12 months, Fair Value	92,114	23,404
Held-to-maturity, Less than 12 months, Unrealized Loss	708	59
Held-to-maturity, 12 months or longer, Fair Value	5,372	5,625
Held-to-maturity, 12 months or longer, Unrealized Loss	110	32
Held-to-maturity, Total, Fair Value	97,486	29,029
Held-to-maturity, Total, Unrealized Loss	$ 818	$ 91