INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 41,806
Due in one through five years	726,690
Due in five through ten years	1,517,966
Due after ten years	237,484
Subtotal	2,523,946
Amortized cost	2,742,028
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	42,150
Due in one through five years	744,452
Due in five through ten years	1,547,677
Due after ten years	249,262
Subtotal	2,583,541
Fair value	2,807,714
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	205,220
Amortized cost	205,220	$ 228,971
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	210,865
Fair value	210,865	237,728
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	12,862
Amortized cost	12,862	17,330
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	13,308
Fair value	$ 13,308	$ 18,025