INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 677,169
Due after one through five years	905,531
Due after five years through ten years	453,320
Due after ten years	494,419
Subtotal	2,530,439
Amortized cost	3,011,551
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	680,805
Due after one through five years	925,987
Due after five years through ten years	456,833
Due after ten years	494,870
Subtotal	2,558,495
Fair value	3,044,428
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	428,491
Amortized cost	428,491	$ 390,234
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	432,862
Fair value	432,862	382,477
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	52,621
Amortized cost	52,621	69,342
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	53,071
Fair value	53,071	68,119
U.S. Treasury Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	2,030,060	2,036,474
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	$ 2,043,759	$ 2,023,753