Workaround For Document.getElementById

I have an onchange method for a select box that goes something like
this (the select is in a form named aForm):

function page_on_change() {
pageElement = aForm.my_page_id;
aForm.nav_page_name.value =
pages[pageElement.options[ pageElement.selectedIndex ].value];
var si = pageElement.selectedIndex;
for ( i = 0 ; i < pages.size ; i++ ) {
document.getElementById("id_" + i).style.display='none'
}
document.getElementbyId("id_" + si).style.display='none'
}

IE fails on the statement:

document.getElementbyId("id_" + si).style.display='none'

and says it doesn't support this property.

What is the workaround?