SECURITIES - Amortized Cost and Estimated Fair Value of Investments in Debt Securities By Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available for sale, Amortized Cost
Due in one year or less	$ 3,559
Due after one year through five years	39,169
Due after five years through ten years	176,316
Due after ten years	512,098
Total available-for-sale investment securities	731,142
Mortgage-backed Securities	1,282,343
Amortized cost	2,013,485
Available for sale Securities, Fair Value
Due in one year or less	3,602
Due after one year through five years	40,055
Due after five years through ten years	175,181
Due after ten years	500,472
Subtotal	719,310
Mortgage-backed Securities and Other Equity Securities	1,270,126
Total	1,989,436
Held to Maturity, Amortized Cost
Due in one year or less	116
Due after one year through five years	1,696
Due after five years through ten years	1,271
Due after ten years	0
Subtotal	3,083
Mortgage-backed Securities	159,848
Amortized cost	162,931	$ 909,506
Held to Maturity, Fair Value
Due in one year or less	115
Due after one year through five years	1,674
Due after five years through ten years	1,257
Due after ten years	0
Subtotal	3,046
Mortgage-backed Securities	156,735
Total	$ 159,781	$ 921,800