Passing value from one page to another page is not working in oracle apex

Question

In Page 20 i am having a grid which is a checkbox, i am just selecting the checkbox value and storing as : separated in one item :p20_doc_id and passing that item to next page when UPDATE button clicked through Dynamic action, but if i have selected 3 document id its just taking 2

Here now item hold value like :P20_DOC_ID == 20:21:22:23:25 if i have selected 5 document 

Below is the js code:

  BQ.customConfirm( "Do you want to update selected document?", function( okPressed ) {
  if (okPressed) {
  var DocumentId  = apex.item('P20_DOC_ID').getValue();
  BQ.openPage(200, 3, {items:'P3_DOCUMENT_ID', values: DocumentId,
  triggering_element: "apex.jQuery('#not_required')", clone_session: 'false',
  mode: 'js' });
  };
  }, "Yes", "No");