INCOME TAXES - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
Maximum corporate tax rate			37.50%	37.50%	39.00%
Net deferred tax asset			$ 329,261	$ 264,842
Reversal of deferred tax asset valuation allowance			(8,000)	0	$ (63,228)
Deferred tax asset related to NOLs			144,700
Valuation allowance			101,984	86,553
Deferred tax asset not subject to expiration period			210,700	92,000
Income tax expense (benefit)			14,050	71,995	$ (10,970)
UTBs			1,000
ASC 326 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Income Tax Contingency [Line Items]
Increase in Deferred Tax Assets	$ 31,300
Santander [Member]
Income Tax Contingency [Line Items]
Net deferred tax asset	$ 28,900
Reversal of deferred tax asset valuation allowance		$ (8,000)
UTBs			1,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense			117
U S [Member]
Income Tax Contingency [Line Items]
Income tax expense (benefit)			4,900	4,500
FirstBank [Member]
Income Tax Contingency [Line Items]
Net deferred tax asset			329,100	264,800
Valuation allowance			59,900	$ 55,600
Holding Company and First Management [Member]
Income Tax Contingency [Line Items]
Valuation allowance			$ 43,000
Domestic Country [Member] | Subsidiaries [Member]
Income Tax Contingency [Line Items]
Dividend received deduction			100.00%
Domestic Country [Member] | Other taxable domestic corporations [Member]
Income Tax Contingency [Line Items]
Dividend received deduction			85.00%