INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 58,214
Due after one through five years	2,126,742
Due after five years through ten years	158,282
Due after ten years	149,464
Subtotal	2,492,702
Amortized cost	2,952,278
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	58,128
Due after one through five years	2,115,304
Due after five years through ten years	160,694
Due after ten years	151,327
Subtotal	2,485,453
Fair value	2,936,049
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	390,234
Amortized cost	390,234	$ 358,180
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	382,477
Fair value	382,477	357,538
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	69,342
Amortized cost	69,342	88,097
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	68,119
Fair value	68,119	87,374
U.S. Treasury Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	2,036,474	2,043,023
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	$ 2,023,753	$ 2,054,071