INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 76,679
Due after one through five years	1,489,433
Due after five years through ten years	143,339
Due after ten years	156,030
Subtotal	1,865,481
Amortized cost	2,322,697
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	76,659
Due after one through five years	1,513,356
Due after five years through ten years	147,980
Due after ten years	161,334
Subtotal	1,899,329
Fair value	2,359,038
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	395,220
Amortized cost	395,220	$ 390,234
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	397,195
Fair value	397,195	382,477
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	61,996
Amortized cost	61,996	69,342
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	62,514
Fair value	$ 62,514	$ 68,119