You can utilize the following FlexReport query to retrieve instance level cost categorized by the usage for Document DB with the below query:
{
"sqlStatement": "SELECT timeInterval_Month AS Month, SUM(lineItem_UnblendedCost) AS SUM_lineItem_UnblendedCost, product_ProductName AS Product_ProductName, product_productFamily AS Product_productFamily, lineItem_ResourceId AS LineItem_ResourceId FROM AWS_CUR WHERE (product_ProductName LIKE '%Amazon DocumentDB%') GROUP BY timeInterval_Month, product_ProductName, product_productFamily, lineItem_ResourceId",
"needBackLinkingForTags": true,
"dataGranularity": "MONTHLY",
"timeRange": {
"last": 3
},
"limit": -1
}