INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Held-to-Maturity Amortized Cost
Amortized Cost	$ 154,327	$ 429,328
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	153,039	424,118
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	3,104,152	2,792,326
Available-for-Sale Market Value
Investment securities available-for-sale	3,152,970	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	8,322
Available-for-Sale Market Value
Investment securities available-for-sale	8,337
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	41,902
Available-for-Sale Market Value
Investment securities available-for-sale	42,607
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,284
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	4,721
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	156,370
Available-for-Sale Market Value
Investment securities available-for-sale	160,519
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	7,233
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	7,281
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	446,306
Available-for-Sale Market Value
Investment securities available-for-sale	461,386
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	23,736	25,565
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	23,705	24,520
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	507,434	565,071
Available-for-Sale Market Value
Investment securities available-for-sale	513,989	558,599
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	107,810	147,780
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	106,249	143,653
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	482,157	423,797
Available-for-Sale Market Value
Investment securities available-for-sale	486,645	421,035
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	11,264	12,540
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	11,083	11,907
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	940,304	928,586
Available-for-Sale Market Value
Investment securities available-for-sale	958,386	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	521,357	511,430
Available-for-Sale Market Value
Investment securities available-for-sale	$ 521,101	$ 509,231