INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 201,210
Due after one through five years	727,189
Due after five years through ten years	954,210
Due after ten years	1,442,839
Subtotal	3,325,448
Amortized cost	3,898,704
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	202,974
Due after one through five years	758,445
Due after five years through ten years	952,781
Due after ten years	1,295,311
Subtotal	3,209,511
Fair value	3,791,391
Government agency mortgage-backed securities
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	537,317
Amortized cost	537,317	$ 506,540
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	544,986
Fair value	544,986	522,638
Government agency collateralized mortgage obligations
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	35,939
Amortized cost	35,939	42,476
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	36,894
Fair value	$ 36,894	$ 43,577