Long-Term Debt - Narrative, Part II (Details) $ in Millions	Dec. 31, 2015 USD ($)
Long-Term Debt
Trust preferred securities included in Tier 1 capital	$ 70.0

Junior Subordinated Debenture, Owed To Unconsolidated Subsidiary Trust, Portion Included in Tier I Regulatory Capital

No definition available.

Name:	cobz_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrustPortionIncludedInTierIRegulatoryCapital
Namespace Prefix:	cobz_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant