BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Detail) $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) numberofcontracts	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) numberofcontracts	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Goodwill		$ 34,401		$ 34,401		$ 28,098
Increase (decrease) in ACL				229,600
Provision for credit losses		46,914	$ 7,398	163,294	$ 31,340
Loans and Leases Receivable, Gross		$ 11,800,000		$ 11,800,000
Excess Cash And Liquid Securities Total Assets Ratio		19.20%		19.20%		15.80%
Federal Home Loan Bank Advances General Debt Obligations Disclosures Amount Of Available Unused Funds		$ 1,400		$ 1,400
Federal Reserve Bank Amount Of Available Unused Funds		$ 936,100		936,100
COVID-19 | Small Business Administration
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
SBA PPP number of applications | numberofcontracts		6,000
SBA loans approved		$ 390,300		390,300
COVID-19 | Payment Deferral [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans and Leases Receivable, Gross		1,200,000		$ 1,200,000
Percentage Of Deferred Payments To Total Portfolio				10.00%
COVID-19 | Residential Portfolio Segment [Member] | Payment Deferral [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan Deferral Number Of Contract | numberofcontracts				3,227
Loans and Leases Receivable, Gross		511,900		$ 511,900
COVID-19 | Commercial Portfolio Segment [Member] | Payment Deferral [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan Deferral Number Of Contract | numberofcontracts				196
Loans and Leases Receivable, Gross		540,800,000		$ 540,800,000
COVID-19 | Consumer Portfolio Segment [Member] | Payment Deferral [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loan Deferral Number Of Contract | numberofcontracts				21,750
Loans and Leases Receivable, Gross		$ 168,700		$ 168,700
Santander [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payments To Acquire Businesses Gross Core Tangible Common Equity	$ 394,800
Business Acquisition Purchase Price To Tangible Common Equity	117.50%
Payments To Acquire Businesses Gross Acquisition Premium	$ 58,800
Tangible Common Equity	336,000
Payments To Acquire Businesses Gross Excess Capital	882,800
Total assets and identifiable intangible assets acquired	$ 5,500,000
Number Of Branches Acquired	27
Goodwill	$ 6,300
Santander [Member] | Fair Value
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total assets and identifiable intangible assets acquired	5,562,132
Total liabilities assumed	4,290,809
Goodwill	6,303
Santander [Member] | Loans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total assets and identifiable intangible assets acquired	2,600,000
Santander [Member] | Deposits [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total liabilities assumed	$ 4,200,000