According to a developer familiar with reporting, the .reporting indices should only have documents with type esqueue in it. Do you have any idea how a document with type doc could have gotten into the index?
You could check how many documents are in the index in the Dev Tools > Console like;
GET .reporting-2018.01.21/_count
Then you can get the actual documents like;
GET .reporting-2018.01.21/_search
In my results after I generated 2 reports I see 2 documents with "_type": "esqueue"
If you don't need any reports that have been generated since 2018-01-21 you could delete that whole index and reporting would recreate it the next time you generate a report.
Or, you could delete the documents with "_type": "doc" but that would take a minute for me to figure out and test so let me know if you need to keep some existing reports.
Actually, now that I think about it, you can't have any actual reports in that index because they all would have gotten that error. So I expect it would be save to delete the whole index.