Bulk updating data in DocumentDB

You can use Azure Document DB Studio as a front end to creating and executing a stored procedure. It can be found here. It's pretty easy to setup and use.

I've mocked up a stored procedure based on your example:

function updateArticlesDetailsX() {

   var collection = getContext().getCollection();
   var collectionLink = collection.getSelfLink();
   var response = getContext().getResponse();
   var docCount = 0;
   var counter = 0;

   tryQueryAndUpdate();

   function tryQueryAndUpdate(continuation) {
        var query = {
            query: "select * from root r where IS_DEFINED(r.details.x) != true"
        };

        var requestOptions = {
            continuation: continuation
        };

        var isAccepted =
            collection
            .queryDocuments(collectionLink,
                            query,
                            requestOptions,
                            function queryCallback(err, documents, responseOptions) {
                                     if (err) throw err;
                                     if (documents.length > 0) {
                                        // If at least one document is found, update it.
                                        docCount = documents.length;
                                        for (var i=0; i<docCount; i++){
                                            tryUpdate(documents[i]);
                                        }
                                        response.setBody("Updated " + docCount + " documents");
                                      }
                                      else if (responseOptions.continuation) {
                                          // Else if the query came back empty, but with a continuation token; 
                                          // repeat the query w/ the token.
                                        tryQueryAndUpdate(responseOptions.continuation);
                                      } else {
                                             throw new Error("Document not found.");
                                             }
                            });

        if (!isAccepted) {
            throw new Error("The stored procedure timed out");
        }
    }

    function tryUpdate(document) {
        //Optimistic concurrency control via HTTP ETag.
        var requestOptions = { etag: document._etag };

        //Update statement goes here:
        document.details.x = "some new value";

        var isAccepted = collection
                         .replaceDocument(document._self,
                                          document,
                                          requestOptions,
                                          function replaceCallback(err, updatedDocument, responseOptions) {
                                                   if (err) throw err;
                                                   counter++;
                                           });

        // If we hit execution bounds - throw an exception.
        if (!isAccepted) {
            throw new Error("The stored procedure timed out");
        }
    }
}

I got the rough outline for this code from Andrew Liu on GitHub.

This outline should be close to what you need to do.