cm:Concept: us-gaap:IncrementalCommonSharesAttributableToNonvestedSharesWithForfeitableDividends
| Incremental Common Shares Attributable to Dilutive Effect of Nonvested Shares with Forfeitable Dividends |
| us-gaap:IncrementalCommonSharesAttributableToNonvestedSharesWithForfeitableDividends |
| Number of additional shares included in diluted EPS for potentially dilutive effect of nonvested equity-based payment award containing forfeitable rights to dividends or dividend equivalents, whether paid or unpaid. |
| cm:Concept |
| Shares (xbrli:sharesItemType) |
| For Period (duration) |
| (no balance type) |
| false |
| us-gaap:IncrementalCommonSharesAttributableToNonvestedSharesWithForfeitableDividends |