Например
<DataGrid RowDetailsVisibilityMode="Collapsed">
<DataGrid.RowStyle>
<Style TargetType="{x:Type DataGridRow}">
<EventSetter Event="MouseDoubleClick" Handler="RowDoubleClick"/>
</Style>
</DataGrid.RowStyle>
</DataGrid>
private void RowDoubleClick(object sender, RoutedEventArgs e)
{
var row = (DataGridRow)sender;
row.DetailsVisibility = row.DetailsVisibility == Visibility.Collapsed ?
Visibility.Visible : Visibility.Collapsed;
}