INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 63,241
Due after one through five years	1,884,323
Due after five years through ten years	414,137
Due after ten years	168,132
Subtotal	2,529,833
Amortized cost	2,972,889
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	63,194
Due after one through five years	1,861,933
Due after five years through ten years	412,375
Due after ten years	170,507
Subtotal	2,508,009
Fair value	2,939,944
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	364,919
Amortized cost	364,919	$ 358,180
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	355,897
Fair value	355,897	357,538
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	78,137
Amortized cost	78,137	88,097
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	76,038
Fair value	76,038	87,374
U.S. Treasury Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	2,042,116	2,043,023
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	2,013,962	$ 2,054,071
Securities pledged as collateral for securities sold under agreement to repurchase	$ 181,800