Use LINQ XML with a namespace as attribute

I am trying to find nodes in an XML document like this:

<?xml version="1.0"?>
<TrainingCenterDatabase xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY" xmlns="http://ift.tt/VxNYPL">
  <Activities>
    <Activity Sport="CyclingTransport">
      <Id>2014-07-08T15:28:14Z</Id>
    </Activity>
  </Activities>
</TrainingCenterDatabase>

I aim to extract the node value 'Id' with code like this:

XDocument doc = XDocument.Load(filePath);
List<string> urlList = doc.Root.Descendants("Id")
                          .Select(x => (string)x)
                          .ToList();
Console.WriteLine(urlList.Count);

However the count is 0, where I expect 1.

After some debugging and editing the XML I noticed that if I change the TrainingCenterDatabase node and remove the attributes to this:

<TrainingCenterDatabase>

Then the result is a count of 1 as expected.

So my question is how do I take into account the namespaces so that I can get the value when the TrainingCenterDatabase node has these attributes?