cm:Axis: us-gaap:SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
| Security Owned and Sold, Not yet Purchased, at Fair Value [Axis] |
| us-gaap:SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis |
| Group of financial instruments held by a broker-dealer for their own account (proprietary securities) for trading or investment purposes that are carried at fair value including securities sold, not yet purchased. |
| cm:Axis |
| Text/String (xbrli:stringItemType) |
| For Period (duration) |
| (no balance type) |
| true |
| us-gaap:SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis |