INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale Debt Securities
Amortized Cost	$ 933,678	$ 847,791
Gross Unrealized Gain	228	1,356
Gross Unrealized Loss	26,088	11,648
Investment securities available for sale	907,818	837,499
Held-to-Maturity Securities
Amortized Cost	159,672	161,186
Fair Value	159,164	162,853
Equity Investments
Amortized Cost	18,047	17,991
Gross Unrealized Gain	15,346	0
Gross Unrealized Loss	30	20
Fair Value	33,363	17,971
CMO
Available-for-Sale Debt Securities
Amortized Cost	277,079	250,592
Gross Unrealized Gain	5	88
Gross Unrealized Loss	7,848	4,141
Investment securities available for sale	269,236	246,539
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	523,676	479,218
Gross Unrealized Gain	71	941
Gross Unrealized Loss	14,747	6,172
Investment securities available for sale	509,000	473,987
FHLMC MBS
Available-for-Sale Debt Securities
Amortized Cost	99,860	88,681
Gross Unrealized Gain	3	118
Gross Unrealized Loss	2,564	924
Investment securities available for sale	97,299	87,875
GNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	33,063	29,300
Gross Unrealized Gain	149	209
Gross Unrealized Loss	929	411
Investment securities available for sale	32,283	29,098
Other investments
Equity Investments
Amortized Cost	3,947	3,943
Gross Unrealized Gain	0	0
Gross Unrealized Loss	30	20
Fair Value	3,917	3,923
State and political subdivisions
Held-to-Maturity Securities
Amortized Cost	159,672	161,186
Gross Unrealized Gain	310	1,758
Gross Unrealized Loss	818	91
Fair Value	159,164	162,853
Equity Investments
Available for sale securities transfers to held to maturity unrealized gains	1,500	1,600
Common Class B | Visa Class B shares | Common Stock
Equity Investments
Amortized Cost	14,100	14,048
Gross Unrealized Gain	15,346	0
Gross Unrealized Loss	0	0
Fair Value	$ 29,446	$ 14,048