INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 76,873
Due after one through five years	2,093,361
Due after five years through ten years	152,168
Due after ten years	137,485
Subtotal	2,459,887
Amortized cost	2,915,267
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	76,767
Due after one through five years	2,096,910
Due after five years through ten years	156,042
Due after ten years	140,965
Subtotal	2,470,684
Fair value	2,922,943
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	389,154
Amortized cost	389,154	$ 390,234
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	386,543
Fair value	386,543	382,477
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	66,226
Amortized cost	66,226	69,342
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	65,716
Fair value	$ 65,716	$ 68,119