INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 41,579
Due in one to five years	166,945
Due in five through ten years	2,077,658
Due after ten years	247,411
Subtotal	2,533,593
Amortized Cost	2,760,772
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	41,884
Due in one to five years	171,082
Due in five through ten years	2,117,573
Due after ten years	252,487
Subtotal	2,583,026
Fair value	2,817,436
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	212,145
Amortized Cost	212,145	$ 228,971
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	218,755
Fair value	218,755	237,728
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	15,034
Amortized Cost	15,034	17,330
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	15,655
Fair value	$ 15,655	$ 18,025