INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Held-to-Maturity Amortized Cost
Amortized Cost	$ 148,778	$ 429,328
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	149,724	424,118
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,789,151	2,792,326
Available-for-Sale Market Value
Investment securities available-for-sale	2,850,502	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	8,694
Available-for-Sale Market Value
Investment securities available-for-sale	8,713
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	48,276
Available-for-Sale Market Value
Investment securities available-for-sale	49,127
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,321
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	4,830
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	147,357
Available-for-Sale Market Value
Investment securities available-for-sale	152,333
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	6,959
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	7,094
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	463,307
Available-for-Sale Market Value
Investment securities available-for-sale	483,955
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	22,367	25,565
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	22,510	24,520
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	416,446	565,071
Available-for-Sale Market Value
Investment securities available-for-sale	426,022	558,599
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	104,586	147,780
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	104,863	143,653
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	440,457	423,797
Available-for-Sale Market Value
Investment securities available-for-sale	445,334	421,035
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	10,545	12,540
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	10,427	11,907
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	799,072	928,586
Available-for-Sale Market Value
Investment securities available-for-sale	818,931	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	465,542	511,430
Available-for-Sale Market Value
Investment securities available-for-sale	$ 466,087	$ 509,231

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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