INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 43,083
Due after one through five years	1,507,979
Due after five years through ten years	832,917
Due after ten years	200,314
Subtotal	2,584,293
Amortized cost	3,021,053
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Due in one year or less	43,270
Due after one through five years	1,531,059
Due after five years through ten years	852,985
Due after ten years	207,448
Subtotal	2,634,762
Fair value	3,073,733
Government Agency Mortgage-Backed Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	342,838
Amortized cost	342,838	$ 232,657
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	344,922
Fair value	344,922	235,230
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Without single maturity date	93,922
Amortized cost	93,922	9,225
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Without single maturity date	94,049
Fair value	94,049	9,535
U.S. Treasury Securities [Member]
Available-for-Sale, Debt Maturities, Amortized Cost [Abstract]
Amortized cost	2,040,820	1,876,358
Available-for-Sale, Debt Maturities, Fair Value [Abstract]
Fair value	2,074,141	$ 1,902,762
Securities pledged as collateral for securities sold under agreement to repurchase	$ 310,700