INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 662,404
Due after one through five years	893,934
Due after five years through ten years	445,987
Due after ten years	502,442
Subtotal	2,504,767
Amortized cost	2,986,577
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	671,422
Due after one through five years	940,096
Due after five years through ten years	485,140
Due after ten years	546,637
Subtotal	2,643,295
Fair value	3,141,882
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	433,129
Amortized cost	433,129	$ 428,491
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	448,248
Fair value	448,248	432,862
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	48,681
Amortized cost	48,681	52,621
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	50,339
Fair value	$ 50,339	$ 53,071