INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 53,102
Due after one through five years	1,557,644
Due after five years through ten years	789,557
Due after ten years	149,642
Subtotal	2,549,945
Amortized cost	3,004,915
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	53,115
Due after one through five years	1,546,876
Due after five years through ten years	788,557
Due after ten years	152,253
Subtotal	2,540,801
Fair value	2,986,769
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	371,857
Amortized cost	371,857	$ 358,180
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	364,539
Fair value	364,539	357,538
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	83,113
Amortized cost	83,113	88,097
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	81,429
Fair value	81,429	87,374
U.S. Treasury Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	2,044,795	2,043,023
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	2,027,914	$ 2,054,071
Securities pledged as collateral for securities sold under agreement to repurchase	$ 279,700