INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 663,860
Due after one through five years	925,191
Due after five years through ten years	466,983
Due after ten years	496,635
Subtotal	2,552,669
Amortized cost	3,129,978
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	670,059
Due after one through five years	971,561
Due after five years through ten years	509,723
Due after ten years	547,242
Subtotal	2,698,585
Fair value	3,293,116
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	519,370
Amortized cost	519,370	$ 428,491
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	535,078
Fair value	535,078	432,862
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	57,939
Amortized cost	57,939	52,621
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	59,453
Fair value	$ 59,453	$ 53,071