The Course ID is unique in an instance of Canvas. From the custom_ prefix I guess you are getting ID through a LTI launch? If so there's documentation on the LTI launch variables at: https://canvas.instructure.com/doc/api/file.tools_variable_substitutions.html. The Course ID is often seen in course URLs when using Canvas (eg https://canvas.instructure.com/courses/1234). However watch out as if someone uses the "Reset Course Content" feature in Canvas, the course will get re-created with a new Course ID.
