Applying DocumentListener on multiple JTextField
Almost fine. You are doing the same stuff in all 3 update methods of the DocumentListener, so I suggest creating a new private method in it and just calling it from all 3 update methods. That way you avoid code duplication and only have one location to change in the future instead of having to maintain the same code 3 times.

DocumentListener listener = new DocumentListener() {

    @Override
    public void removeUpdate(DocumentEvent e) {
        doStuff(e);
    }

    @Override
    public void insertUpdate(DocumentEvent e) {
        doStuff(e);
    }

    @Override
    public void changedUpdate(DocumentEvent e) {
        doStuff(e);
    }

    private void doStuff(DocumentEvent e) {
        Object owner = e.getDocument().getProperty("owner");
        enableDetachButton(getContentPane());
        changeColor((JTextField) owner);
    }
});