INVESTMENTS - Estimated fair values of debt securities available-for-sale by contractual maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value
Total available-for-sale debt securities	$ 1,429,555	$ 1,348,021
Amortized Cost
Total available-for-sale debt securities	1,419,020	1,310,159
U.S. treasuries and government agencies
Fair Value
One year or less	39,647	33,963
One to five years	8,082	9,334
Five to ten years	0	0
After ten years	0	0
Total available-for-sale debt securities	47,729	43,297
Amortized Cost
One year or less	39,559	33,833
One to five years	7,918	8,917
Five to ten years	0	0
After ten years	0	0
Total available-for-sale debt securities	47,477	42,750
State and municipal
Fair Value
One year or less	14,173	16,581
One to five years	25,081	44,910
Five to ten years	43,958	59,059
After ten years	244,581	269,817
Total available-for-sale debt securities	327,793	390,367
Amortized Cost
One year or less	14,009	16,458
One to five years	24,229	43,857
Five to ten years	43,207	56,130
After ten years	243,939	260,663
Total available-for-sale debt securities	325,384	377,108
Mortgage-backed and asset-backed
Fair Value
One year or less	7,975	1
One to five years	15,344	21,637
Five to ten years	319,947	74,142
After ten years	703,358	808,652
Total available-for-sale debt securities	1,046,624	904,432
Amortized Cost
One year or less	7,916	1
One to five years	15,063	21,229
Five to ten years	319,621	72,481
After ten years	696,559	787,490
Total available-for-sale debt securities	1,039,159	881,201
Corporate debt
Fair Value
One year or less	0	0
One to five years	0	2,318
Five to ten years	7,409	7,607
After ten years	0	0
Total available-for-sale debt securities	7,409	9,925
Amortized Cost
One year or less	0	0
One to five years	0	2,100
Five to ten years	7,000	7,000
After ten years	0	0
Total available-for-sale debt securities	$ 7,000	$ 9,100