Retirement Plans Retirement Plans (Unfunded Commitments and Redemption) (Details) $ in Millions	Dec. 31, 2018 USD ($)
Retirement Benefits [Abstract]
Retirement Plans, Limited Partnership, Unfunded Commitments	$ 1

Retirement Plans, Limited Partnership, Unfunded Commitments

No definition available.

Name:	zion_RetirementPlansLimitedPartnershipUnfundedCommitments
Namespace Prefix:	zion_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant