SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) $ in Thousands		9 Months Ended	12 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) banking_office	Dec. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of banking offices | banking_office		31
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Impact of adopting ASU 2016-13 on retained earnings		$ (874,254)	$ (907,245)	$ (846,959)	$ (900,988)	$ (963,544)	$ (945,534)
Loans funded		5,678,002	5,951,885		6,383,259
PPP loans | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans funded		255,800
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adopting ASU 2016-13 on retained earnings		$ (95,001)	(127,733)	$ (85,670)	$ (120,221)	$ (146,039)	$ (140,952)
Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to allowance for credit losses	$ 6,400
Impact of adopting ASU 2016-13 on retained earnings			4,503
Impact of ASC 326 Adoption | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adopting ASU 2016-13 on retained earnings			$ 4,503