Use [embedFullWidthRows]="true" when defining ag-grid. Refer here.

Eg:

<ag-grid-angular
    style="width: 100%; height: 100%;"
    [columnDefs]="columnDefs"
    [defaultColDef]="defaultColDef"
    [masterDetail]="true"
    [embedFullWidthRows]="true"
    [detailCellRendererParams]="detailCellRendererParams"
    [rowData]="rowData"
    (firstDataRendered)="onFirstDataRendered($event)"
    (gridReady)="onGridReady($event)"
/>