Add a details group to a paginated report (Report Builder)
Applies to: 
Microsoft Report Builder (SSRS)
Power BI Report Builder
Report Designer in SQL Server Data Tools
In a paginated report, the detail data from a report dataset is specified as a group with no group expression. Add a detail group to an existing tablix data region when you want to display the detail data for a matrix. You can also add back detail data that you deleted from a table or list, or to add more detail groups. For more information about groups, see Understand groups (Report Builder).
Note
You can create and modify paginated report definition (.rdl) files in Microsoft Report Builder, Power BI Report Builder, and in Report Designer in SQL Server Data Tools.
Add a details group to a tablix data region
On the design surface, select a tablix data region. The Grouping pane displays the row and column groups for the selected data region.
In the Grouping pane, right-click a group that is an innermost child group. Select Add Group, and then choose Child Group. The Tablix Group dialog opens.
In Group expression, leave the expression blank. A details group has no expression.
Select Show detail data.
Select OK.
A new details group is added as a child group in the Grouping pane, and the row handle for the group you selected in step 1 displays the details group icon. For more information about handles, see Tablix data region cells, rows, and columns (Report Builder).