INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 38,222
Due after one through five years	1,199,564
Due after five years through ten years	1,041,199
Due after ten years	170,195
Subtotal	2,449,180
Amortized cost	2,694,953
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	38,447
Due after one through five years	1,221,056
Due after five years through ten years	1,058,066
Due after ten years	175,004
Subtotal	2,492,573
Fair value	2,740,436
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	237,257
Amortized cost	237,257	$ 232,657
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	239,054
Fair value	239,054	235,230
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	8,516
Amortized cost	8,516	9,225
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	8,809
Fair value	$ 8,809	$ 9,535