INVESTMENTS - Estimated fair values of debt securities available-for-sale by contractual maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value
Total available-for-sale debt securities	$ 1,357,205	$ 1,072,765
Amortized Cost
Total available-for-sale debt securities	1,323,184	1,067,386
U.S. treasuries and government agencies
Fair Value
One year or less	28,139	69,799
One to five years	11,902	96,709
Five to ten years	0	0
After ten years	99,364	91,987
Amortized Cost
One year or less	27,974	69,330
One to five years	11,380	96,507
Five to ten years	0	0
After ten years	101,100	94,457
State and municipal
Fair Value
One year or less	18,663	33,311
One to five years	47,558	76,723
Five to ten years	59,670	75,820
After ten years	227,096	47,795
Amortized Cost
One year or less	18,457	33,054
One to five years	46,440	75,432
Five to ten years	56,731	73,741
After ten years	222,540	47,082
Mortgage-backed and asset-backed
Fair Value
One year or less	2	852
One to five years	21,191	7,125
Five to ten years	58,878	55,226
After ten years	772,243	507,556
Amortized Cost
One year or less	2	822
One to five years	20,800	6,969
Five to ten years	57,430	54,799
After ten years	748,218	505,783
Corporate debt
Fair Value
One year or less	0	0
One to five years	0	0
Five to ten years	12,499	9,552
After ten years	0	0
Amortized Cost
One year or less	0	0
One to five years	0	0
Five to ten years	12,112	9,100
After ten years	0	0
Trust preferred
Fair Value
One year or less	0	0
One to five years	0	0
Five to ten years	0	0
After ten years	0	310
Amortized Cost
One year or less	0	0
One to five years	0	0
Five to ten years	0	0
After ten years	$ 0	$ 310