function toggle(obj) {
	var el = document.getElementById(obj);
	if ( el.style.display != 'none' ) {
		el.style.display = 'none';
	}
	else {
		el.style.display = '';
	}
}

или

function toggle(obj) {
	var el = document.getElementById(obj);
	el.style.display = (el.style.display != 'none' ? 'none' : '' );
}