INCOME TAXES - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Income Tax Contingency [Line Items]
Maximum corporate tax rate	37.50%
Net deferred tax asset	$ 147,700		$ 150,100
Valuation allowance	140,100		$ 139,200
Income tax expense	$ 23,955	$ 31,935
Minimum percentage of bank net taxable income for paying income tax at normal rate			20.00%
Estimated annual effective tax rate, including all entities	24.30%	31.20%
Santander [Member]
Income Tax Contingency [Line Items]
UTBs	$ 800
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	200
U S [Member]
Income Tax Contingency [Line Items]
Income tax expense	2,200	$ 2,500
Subsidiaries [Member]
Income Tax Contingency [Line Items]
Net deferred tax asset	147,700		$ 150,100
Valuation allowance	112,700		$ 111,400
Parent Company [Member]
Income Tax Contingency [Line Items]
Income tax expense	$ 1	$ 1
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%