Credit Risk Assumptions, Estimated Unemployment Rate

No definition available.

Name:	sov_CreditRiskAssumptionsEstimatedUnemploymentRate
Namespace Prefix:	sov_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Credit Risk Assumptions, Historical Period

+ References

No definition available.

+ Details

Name:	sov_CreditRiskAssumptionsHistoricalPeriod
Namespace Prefix:	sov_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Credit Risk Assumptions, Weighted Used Vehicle Index

+ References

No definition available.

+ Details

Name:	sov_CreditRiskAssumptionsWeightedUsedVehicleIndex
Namespace Prefix:	sov_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing Receivable, Accrued Interest After Allowance For Credit Loss Statement Of Financial Position Extensible List Not Disclosed Flag

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableAccruedInterestAfterAllowanceForCreditLossStatementOfFinancialPositionExtensibleListNotDisclosedFlag
Namespace Prefix:	sov_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing Receivable, Allowance for Credit Loss, Purchased Seasoned Loan and Purchased with Credit Deterioration

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableAllowanceForCreditLossPurchasedSeasonedLoanAndPurchasedWithCreditDeterioration
Namespace Prefix:	sov_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible and reserve for unfunded lending commitments.

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableAllowanceForCreditLossesExcludingAccruedInterestAndOffBalanceSheetCreditLossLiability
Namespace Prefix:	sov_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Financing Receivable, Excluding Accrued Interest, Number Of Deferred Payments Allowed

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableExcludingAccruedInterestNumberOfDeferredPaymentsAllowed
Namespace Prefix:	sov_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Excluding Accrued Interest, Number Of Deferred Payments Per Period

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableExcludingAccruedInterestNumberOfDeferredPaymentsPerPeriod
Namespace Prefix:	sov_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Excluding Accrued Interest, Number Of Payments Required To Be Eligible For Deferred Payment

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableExcludingAccruedInterestNumberOfPaymentsRequiredToBeEligibleForDeferredPayment
Namespace Prefix:	sov_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Purchased Seasoned Loan and Purchased with Credit Deterioration, Amount at Par Value

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivablePurchasedSeasonedLoanAndPurchasedWithCreditDeteriorationAmountAtParValue
Namespace Prefix:	sov_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Financing Receivable, Recorded Investment, Past Due, Percentage Of Payment To Meet Contractual Requirement

+ References

No definition available.

+ Details

Name:	sov_FinancingReceivableRecordedInvestmentPastDuePercentageOfPaymentToMeetContractualRequirement
Namespace Prefix:	sov_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(c)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after allowance for credit loss, of accrued interest on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-5

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3A

+ Details

Name:	us-gaap_FinancingReceivableAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of decrease in allowance for credit loss on financing receivable from sale of loan. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossLoanSold
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of increase (decrease) in allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of allowance for credit loss at acquisition date of financing receivable purchased with credit deterioration. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 19
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-19

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestPurchasedWithCreditDeteriorationAllowanceForCreditLossAtAcquisitionDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Period of increase in weighted-average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount at par value of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 19
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-19

+ Details

Name:	us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationAmountAtParValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of decrease in financing receivable from sale. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-11

+ Details

Name:	us-gaap_FinancingReceivableSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(h))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

Name:	us-gaap_GainLossOnSalesOfLoansNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_CleanUpCallMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_RetailInstallmentContractsAndAutoLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=sov_ExtendedMaturityForNonNaturalDisastersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=sov_ExtendedMaturityForNaturalDisastersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=sov_AllOtherModificationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_MarineAndRVContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

LOANS AND ALLOWANCE FOR CREDIT LOSSES - Narrative (Details) $ in Millions			3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 24, 2026 USD ($)	Jul. 20, 2026 USD ($)	Sep. 30, 2026 USD ($) payment	Jun. 30, 2026 USD ($)	Mar. 31, 2026	Sep. 30, 2025 USD ($)	Sep. 30, 2026 USD ($) payment	Sep. 30, 2025 USD ($)	Dec. 31, 2025 USD ($)
Loans Receivable [Line Items]
Net LHFI			$ 135,058				$ 135,058		$ 76,891
Financing receivable, accrued interest after allowance for credit loss statement of financial position extensible list not disclosed flag					accrued interest receivable		accrued interest receivable
Accrued interest receivable			833				$ 833		$ 588
Financing receivable, loans sold	$ 294	$ 124
Reduction in allowance for credit loss, loans sold			19	$ 31
Reserve on purchased loans at acquisition			856			$ 0	$ 895	$ 0
Loss on sales of loans			$ 21	$ 24
Credit risk assumptions, historical period							3 years
Loans receivable, basis spread on variable rate					215		211
Loans receivable, unemployment rate			4.60%				4.60%		5.50%
Total ACL, end of period			$ 6,762			6,199	$ 6,762	6,199
Financing receivable, excluding accrued interest, allowance for credit loss, period increase (decrease)							$ 709
Percentage of payment needed on past due loans for qualification							90.00%
Gross charge-offs			1,198			1,200	$ 3,435	3,477
Clean-up call
Loans Receivable [Line Items]
Financing receivable, purchased seasoned loan and purchased with credit deterioration, gross purchase amount							205
Reserve on purchased loans at acquisition							39
Financing receivable, purchased with credit deterioration, gross purchase amount							23
Reserve for PCD loans purchased at acquisition date							10
Consumer:
Loans Receivable [Line Items]
Reserve on purchased loans at acquisition			86			0	125	0
Total ACL, end of period			5,458			5,669	5,458	5,669
Financing receivable, excluding accrued interest, allowance for credit loss, period increase (decrease)							(80)
Gross charge-offs			$ 1,136			1,163	3,323	3,359
Consumer: \| RICs and auto loans
Loans Receivable [Line Items]
Gross charge-offs							$ 3,262		$ 4,396
Number of deferred payments allowed \| payment			3				3
Number of deferred payments per period \| payment			1				1
Number of payments required to be eligible for deferred payment \| payment			8				8
Consumer: \| RICs and auto loans \| Extended Maturity For Non-Natural Disasters
Loans Receivable [Line Items]
Term increase from modification (up to)							8 months
Consumer: \| RICs and auto loans \| Extended Maturity For Natural Disasters
Loans Receivable [Line Items]
Term increase from modification (up to)							12 months
Consumer: \| RICs and auto loans \| Payment Deferral Only
Loans Receivable [Line Items]
Term increase from modification (up to)							8 months
Consumer: \| RICs and auto loans \| All Other Modifications
Loans Receivable [Line Items]
Term increase from modification (up to)							36 months
Consumer: \| Marine and RV contracts \| Extended Maturity
Loans Receivable [Line Items]
Term increase from modification (up to)							12 months
Commercial:
Loans Receivable [Line Items]
Reserve on purchased loans at acquisition			$ 770			0	$ 770	0
Total ACL, end of period			1,304			530	1,304	530
Financing receivable, excluding accrued interest, allowance for credit loss, period increase (decrease)							(789)
Gross charge-offs			62			$ 37	112	$ 118	144
Asset Pledged as Collateral
Loans Receivable [Line Items]
Net LHFI			$ 70,100				$ 70,100		$ 50,100