INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 46,761
Due in one to five years	775,417
Due in five through ten years	1,465,017
Due after ten years	211,589
Subtotal	2,498,784
Amortized cost	2,724,981
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	47,048
Due in one to five years	815,276
Due in five through ten years	1,564,831
Due after ten years	227,650
Subtotal	2,654,805
Fair value	2,889,446
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	215,191
Amortized cost	215,191	$ 205,220
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	223,139
Fair value	223,139	210,865
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	11,006
Amortized cost	11,006	12,862
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	11,502
Fair value	$ 11,502	$ 13,308