BASIS OF PRESENTATION - Impact of Adoption (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	$ 8,982,336		$ 9,502,428	$ 9,502,428
Less: Allowance for credit losses on loans	134,414	$ 139,394	63,575	63,575
Allowance for credit losses on securities	96			0
Deferred tax assets, net				51,165
Other liabilities (ACL unfunded loan commitments)	8,593	8,593		100
Retained earnings (deficit)	$ (242,175)			361,082
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans				35
PCD gross up				15,326
Fair value option				(15,291)
Less: Allowance for credit losses on loans		0		25,434
Allowance for credit losses on securities				309
Deferred tax assets, net				8,993
Other liabilities (ACL unfunded loan commitments)				7,993
Retained earnings (deficit)				(24,380)
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans			9,502,463	9,502,463
PCD gross up				15,326
Fair value option				(15,291)
Less: Allowance for credit losses on loans		$ 139,394	$ 89,009	89,009
Allowance for credit losses on securities				309
Deferred tax assets, net				60,158
Other liabilities (ACL unfunded loan commitments)				8,093
Retained earnings (deficit)				$ 336,702