INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 34,640
Due after one through five years	872,160
Due after five years through ten years	1,360,056
Due after ten years	207,953
Subtotal	2,474,809
Amortized cost	2,701,837
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	34,835
Due after one through five years	910,105
Due after five years through ten years	1,440,555
Due after ten years	221,113
Subtotal	2,606,608
Fair value	2,841,845
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	216,942
Amortized cost	216,942	$ 205,220
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	224,733
Fair value	224,733	210,865
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	10,086
Amortized cost	10,086	12,862
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	10,504
Fair value	$ 10,504	$ 13,308