INVESTMENT SECURITIES (Summary of Amount of Gross Unrealized Losses for Debt Securities and Estimated Fair Value) (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	$ 0	$ 3
Available-for-sale, 12 Months or more	1,302	1,504
Available-for-sale, total	1,302	1,507
Available-for-sale, less than 12 Months, estimated fair value	93	390
Available-for-sale, 12 months or more, estimated fair value	7,860	8,230
Available-for-sale, total, estimated fair value	7,953	8,620
U.S. Treasury securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	0	3
Available-for-sale, 12 Months or more	110	116
Available-for-sale, total	110	119
Available-for-sale, less than 12 Months, estimated fair value	0	198
Available-for-sale, 12 months or more, estimated fair value	291	285
Available-for-sale, total, estimated fair value	291	483
Agency securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	20	26
Available-for-sale, total	20	26
Available-for-sale, less than 12 Months, estimated fair value	11	3
Available-for-sale, 12 months or more, estimated fair value	345	403
Available-for-sale, total, estimated fair value	356	406
Agency guaranteed mortgage-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	1,090	1,263
Available-for-sale, total	1,090	1,263
Available-for-sale, less than 12 Months, estimated fair value	12	86
Available-for-sale, 12 months or more, estimated fair value	5,977	6,171
Available-for-sale, total, estimated fair value	5,989	6,257
Small Business Administration loan-backed securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	16	21
Available-for-sale, total	16	21
Available-for-sale, less than 12 Months, estimated fair value	4	35
Available-for-sale, 12 months or more, estimated fair value	343	387
Available-for-sale, total, estimated fair value	347	422
Municipal securities
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	66	78
Available-for-sale, total	66	78
Available-for-sale, less than 12 Months, estimated fair value	51	68
Available-for-sale, 12 months or more, estimated fair value	904	984
Available-for-sale, total, estimated fair value	955	1,052
Other
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Available-for-sale, less than 12 Months	0	0
Available-for-sale, 12 Months or more	0	0
Available-for-sale, total	0	0
Available-for-sale, less than 12 Months, estimated fair value	15	0
Available-for-sale, 12 months or more, estimated fair value	0	0
Available-for-sale, total, estimated fair value	$ 15	$ 0

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.

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479106/326-30-50-4

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AgencySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=zions_AgencyGuaranteedMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=zions_SmallBusinessAdministrationLoanBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=zions_AvailableforsaleUnrealizedlossesOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: