I created one custom button on Opportunity. By clicking that button have to open VF page for that page want to pass the current record ID

window.showModalDialog("/apex/UpdateFields?id="&{!Opportunity.Id},"","width=400; height=200;");

i Kept like this for button. When i am click on that button in the detail page getting "Unexpected token ILLEGAL" in alert box

Here am missing in this syntax??