cm:Concept: us-gaap:OtherComprehensiveLossHeldToMaturitySecurityAdjustmentFromAociForAccretionOfNoncreditPortionOfOttiAfterTax
| Other Comprehensive Loss, Held-to-maturity Security, Adjustment from AOCI for Accretion of Noncredit Portion of OTTI, after Tax |
| us-gaap:OtherComprehensiveLossHeldToMaturitySecurityAdjustmentFromAociForAccretionOfNoncreditPortionOfOttiAfterTax |
| Amount, after tax, of adjustment from accumulated other comprehensive loss for accretion of other than temporary impairment (OTTI) on investment in held-to-maturity security. |
| cm:Concept |
| Monetary (xbrli:monetaryItemType) |
| For Period (duration) |
| Credit |
| false |
| us-gaap:OtherComprehensiveLossHeldToMaturitySecurityAdjustmentFromAociForAccretionOfNoncreditPortionOfOttiAfterTax |