SECURITIES - Amortized Cost and Estimated Fair Value of Investments in Debt Securities By Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available for sale, Amortized Cost
Due in one year or less	$ 2,185
Due after one year through five years	32,972
Due after five years through ten years	90,751
Due after ten years	1,426,346
Total available-for-sale investment securities	1,552,254
Mortgage-backed Securities	885,259
Amortized cost	2,437,513	$ 2,306,741
Available for sale Securities, Fair Value
Due in one year or less	2,223
Due after one year through five years	33,953
Due after five years through ten years	93,430
Due after ten years	1,529,243
Subtotal	1,658,849
Mortgage-backed Securities and Other Equity Securities	928,456
Total	2,587,305	2,358,597
Held to Maturity, Amortized Cost
Due in one year or less	120
Due after one year through five years	509
Due after five years through ten years	278
Due after ten years	0
Subtotal	907
Mortgage-backed Securities	108,091
Amortized cost	108,998
Held to Maturity, Fair Value
Due in one year or less	121
Due after one year through five years	516
Due after five years through ten years	283
Due after ten years	0
Subtotal	920
Mortgage-backed Securities	117,278
Total	$ 118,198	$ 138,879