INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 42,379
Due in one to five years	732,229
Due in five through ten years	1,515,627
Due after ten years	226,947
Subtotal	2,517,182
Amortized Cost	2,737,393
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	42,646
Due in one to five years	765,044
Due in five through ten years	1,594,586
Due after ten years	240,343
Subtotal	2,642,619
Fair value	2,870,326
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	208,300
Amortized Cost	208,300	$ 205,220
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	215,271
Fair value	215,271	210,865
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	11,911
Amortized Cost	11,911	12,862
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	12,436
Fair value	$ 12,436	$ 13,308