try documents.ContentTypes as opposed to the Web.ContentTypes
See also Base Content Type Hierarchy
When you add a site content type to a list or library, SharePoint Foundation makes local copies of any columns that are referenced by the content type and adds them to the list's fields. For more information, see Fields and Field References. In addition, SharePoint Foundation makes a local copy of the site content type and adds the copy to the content type collection on the list. The new list content type is a child of the site content type. The value of the Id property for the list content type is different from the value of the Id property for its parent site content type, but otherwise the two content types are initially the same.