SECURITIES - Amortized Cost and Estimated Fair Value of Investments in Debt Securities By Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available for sale, Amortized Cost
Due in one year or less	$ 1,268
Due after one year through five years	9,140
Due after five years through ten years	40,606
Due after ten years	739,362
Total available-for-sale investment securities	790,376
Mortgage-backed Securities	1,516,365
Amortized cost	2,306,741	$ 2,013,485
Available for sale Securities, Fair Value
Due in one year or less	1,269
Due after one year through five years	9,295
Due after five years through ten years	42,079
Due after ten years	760,296
Subtotal	812,939
Mortgage-backed Securities and Other Equity Securities	1,545,658
Total	2,358,597	1,989,436
Held to Maturity, Amortized Cost
Due in one year or less	115
Due after one year through five years	1,663
Due after five years through ten years	1,110
Due after ten years	0
Subtotal	2,888
Mortgage-backed Securities	131,975
Amortized cost	134,863	162,931
Held to Maturity, Fair Value
Due in one year or less	116
Due after one year through five years	1,678
Due after five years through ten years	1,124
Due after ten years	0
Subtotal	2,918
Mortgage-backed Securities	135,961
Total	$ 138,879	$ 159,781