Node.ownerDocument

The ownerDocument read-only property of the Node interface returns the top-level document object of the node.

var document = element.ownerDocument;

document is the top-level Document object in which all the child nodes are created. If this property is used on a node that is itself a document, the value is null.

// Given a node "p", get the top-level HTML
// child of the document object

var d = p.ownerDocument;
var html = d.documentElement;