INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale, Debt Maturities, Amortized Cost
Due in one year or less	$ 760,019
Due after one through five years	802,750
Due after five years through ten years	461,012
Due after ten years	485,753
Subtotal	2,509,534
Amortized cost	3,068,406
Available-for-Sale, Debt Maturities, Fair Value
Due in one year or less	765,009
Due after one through five years	843,487
Due after five years through ten years	503,778
Due after ten years	534,994
Subtotal	2,647,268
Fair value	3,222,722
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost
Without single maturity date	508,738
Amortized cost	508,738	$ 428,491
Available-for-Sale, Debt Maturities, Fair Value
Without single maturity date	523,974
Fair value	523,974	432,862
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost
Without single maturity date	50,134
Amortized cost	50,134	52,621
Available-for-Sale, Debt Maturities, Fair Value
Without single maturity date	51,480
Fair value	$ 51,480	$ 53,071