Amount of increase (decrease) in allowance for loan and lease losses write-offs, net.

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ALLOWANCE FOR CREDIT LOSSES ON LOANS AND FINANCE LEASES - Additional Information (Detail) - USD ($) $ in Thousands			3 Months Ended	12 Months Ended
	Sep. 01, 2020	Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019	Jan. 01, 2018	Dec. 31, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses			$ 385,887	$ 385,887	$ 155,139	$ 196,362			$ 231,843
Increase (decrease) in ACL				230,800
Provision (release) for credit losses				168,717	40,225	59,253
Allowance established for acquired PCD loans	$ 28,744	$ 28,700
Increase (decrease) in provision for credit losses				128,500
Net charge-offs				47,900	81,400
Recoveries				16,662	16,139	22,058
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)				1,183	(412)	(264)
Off-Balance Sheet, Credit Loss, Liability			5,105	5,105	0	412			676
Residential Portfolio Segment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses			120,311	120,311	44,806	50,794			58,975
Provision (release) for credit losses				22,427	14,091	13,202
Allowance established for acquired PCD loans	12,739
Increase (decrease) in net charge-offs				10,600
Recoveries				1,519	2,663	3,392
Commercial Portfolio Segment [Member] \| Nonaccrual loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Recoveries			3,900
Commercial Portfolio Segment [Member] \| Mortgage Receivable [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses			109,342	109,342	39,194	55,581			48,493
Provision (release) for credit losses				81,125	(1,697)	23,074
Allowance established for acquired PCD loans	9,723
Recoveries				1,936	398	7,925
Commercial Portfolio Segment [Member] \| Mortgage Receivable [Member] \| U S [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Net charge-offs					11,400
Consumer Portfolio Segment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses			$ 112,910	112,910	53,571	53,849			$ 70,982
Provision (release) for credit losses				56,433	43,023	24,385
Allowance established for acquired PCD loans	4,452
Increase (decrease) in net charge-offs				6,600
Recoveries				$ 9,831	$ 8,859	$ 8,588
Non-PCD \| Santander [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Provision (release) for credit losses	$ 37,500
ASC 326 [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		236,304
Off-Balance Sheet, Credit Loss, Liability		3,922
ASC 326 [Member] \| Residential Portfolio Segment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		94,643
ASC 326 [Member] \| Commercial Portfolio Segment [Member] \| Mortgage Receivable [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		19,888
ASC 326 [Member] \| Consumer Portfolio Segment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		88,677
ASC 326 [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		81,165
Increase (decrease) in ACL		(434)
Impact of adopting ASC 326 in unfunded loan commitments and standby letters of credit		3,922					$ 0	$ 0
Off-Balance Sheet, Credit Loss, Liability		3,922
ASC 326 [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member] \| Residential Portfolio Segment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		49,837
ASC 326 [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member] \| Commercial Portfolio Segment [Member] \| Mortgage Receivable [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		(19,306)
ASC 326 [Member] \| Cumulative Effect, Period of Adoption, Adjustment [Member] \| Consumer Portfolio Segment [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses		$ 35,106