INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Held-to-Maturity Amortized Cost
Amortized Cost	$ 158,305	$ 429,328
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	153,075	424,118
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	3,096,085	2,792,326
Available-for-Sale Market Value
Investment securities available-for-sale	3,113,811	2,779,255
One Year or Less [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	18,503
Available-for-Sale Market Value
Investment securities available-for-sale	18,522
After One Year Through Five Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	52,079
Available-for-Sale Market Value
Investment securities available-for-sale	52,547
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,247
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	4,579
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	162,531
Available-for-Sale Market Value
Investment securities available-for-sale	165,532
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	7,359
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	7,239
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	419,127
Available-for-Sale Market Value
Investment securities available-for-sale	427,878
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	24,937	25,565
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	24,338	24,520
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	557,730	565,071
Available-for-Sale Market Value
Investment securities available-for-sale	558,847	558,599
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	109,826	147,780
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	105,412	143,653
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	464,611	423,797
Available-for-Sale Market Value
Investment securities available-for-sale	463,444	421,035
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	11,936	12,540
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	11,507	11,907
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	919,486	928,586
Available-for-Sale Market Value
Investment securities available-for-sale	925,633	926,747
Asset-backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	502,018	511,430
Available-for-Sale Market Value
Investment securities available-for-sale	$ 501,408	$ 509,231