Derivative Instruments and Hedging Activities (Derivative gain (loss) recognized/reclassified) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) reclassified from AOCI into income	$ 46	$ 36	$ (5)
Gains to be reclassified within the next 12 months into interest income	32	61
Derivatives designated as hedges
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	(34)	101
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	62	47
Interest on fair value hedges	15	18
Hedge ineffectiveness / AOCI reclass due to missed forecast	0	0
Derivatives designated as hedges \| Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Terminated or redesignated, unamortized basis adjustments	7	7
Derivatives designated as hedges \| Cash Flow Hedging \| Purchased interest rate floors
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	0	0
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	11	11
Interest on fair value hedges	0	0
Hedge ineffectiveness / AOCI reclass due to missed forecast	0	0
Derivatives designated as hedges \| Cash Flow Hedging \| Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	(34)	101
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	51	36
Interest on fair value hedges	0	0
Hedge ineffectiveness / AOCI reclass due to missed forecast	0	0
Derivatives designated as hedges \| Fair Value Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Terminated or redesignated, unamortized basis adjustments	1	12
Derivatives designated as hedges \| Fair Value Hedging \| Receive-fixed interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	0	0
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	0	0
Interest on fair value hedges	8	6
Hedge ineffectiveness / AOCI reclass due to missed forecast	0	0
Derivatives designated as hedges \| Fair Value Hedging \| Basis amortization on terminated hedges, liability hedges
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	0	0
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	0	0
Interest on fair value hedges	10	13
Hedge ineffectiveness / AOCI reclass due to missed forecast	0	0
Derivatives designated as hedges \| Fair Value Hedging \| Pay-fixed interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	0	0
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	0	0
Interest on fair value hedges	(3)	(1)
Hedge ineffectiveness / AOCI reclass due to missed forecast	0	0
Derivatives designated as hedges \| Fair Value Hedging \| Basis amortization on terminated hedges, asset hedges
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion of derivative gain/(loss) deferred in AOCI	0	0
Excluded components deferred in AOCI (amortization approach)	0	0
Amount of gain/(loss) reclassified from AOCI into income	0	0
Interest on fair value hedges	0	0
Hedge ineffectiveness / AOCI reclass due to missed forecast	$ 0	$ 0

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

Name:	us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

No definition available.

+ Details

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

- Definition

The amount of net gain (loss) reclassified into earnings when interest rate cash flow hedge is discontinued because it is probable that the original forecasted transactions will not occur by the end of the original period or an additional two month time period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994

+ Details

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

- Definition

Amount, before tax and reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121577181&loc=SL110061190-113977

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624171-113959

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669646-108580

+ Details

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

- Definition

Amount, after tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e689-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580

+ Details

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

- Definition

Amount, before tax, of adjustments from accumulated other comprehensive income (AOCI) for gain (loss) from increase (decrease) in value of excluded component of derivative designated and qualifying as hedge. Adjustments include, but are not limited to, reclassifications for sale and settlement, and amounts recognized under systematic and rational method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (dd)
-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e689-108580

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624171-113959

+ Details

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

- Definition

Derivative instrument noninterest income (expense)

+ References

No definition available.

+ Details

Name:	zions_DerivativeInstrumentNoninterestIncomeExpense
Namespace Prefix:	zions_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Terminated And Redesignated Swaps, Unamortized Basis Adjustments

+ References

No definition available.

+ Details

Name:	zions_TerminatedAndRedesignatedSwapsUnamortizedBasisAdjustments
Namespace Prefix:	zions_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateFloorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=zions_TerminatedSwapsOnLongTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=zions_AssetHedgePayFixedInterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=zions_TerminatedSwapsOnAssetHedgesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: