WPF: Set not being hit on 2way binding

I have an IEnumerable on twoway binding. When selected it populates the element info into a text box. However none of these edits triggers the set operation for my binded object.

C#

private  IEnumerable<XElement> _Steps
    {
        get
        {
            if (_Xdoc== null) return null; 
            else 
                return (_Xdoc.Descendants().Where(x => x.Name == ns + "Implementation").Elements()); 
            }
        }
        set
        {
            XElement xNode = _Steps.Elements().Except(value).First();
            XElement replaceNode = value.Elements().Except(_Steps).First();
            _Xdoc.ToString().Replace(xNode.ToString(), value.ToString());
        }
        }

    public IEnumerable<XElement> Steps { 
        get { return _Steps; } 
        set { _Steps = value; } }

XAML

    <ListView HorizontalAlignment="Stretch"   Name="listView1" VerticalAlignment="Top"  Grid.Column="0" Grid.Row="1"
       ItemsSource="{Binding Steps, Mode=TwoWay  }" Grid.RowSpan="3">
        <ListView.View>
            <GridView>

                <GridViewColumn Header="Name" DisplayMemberBinding="{Binding Path=Attribute[name].Value}"   />
            </GridView>
        </ListView.View>
    </ListView>
    <GridSplitter HorizontalAlignment="Left" Height="100" VerticalAlignment="Top" Width="6" Grid.Column="1" Margin="2.6,80,0,0" Grid.Row="2"/>
    <WrapPanel Grid.Row="1"  Grid.Column="1">
        <Label Content="StepID:" Grid.Column="1" HorizontalAlignment="Left" Grid.Row="1" VerticalAlignment="Top" />
        <Label Content="{Binding Path=Attribute[stepID].Value}" HorizontalAlignment="Left" VerticalAlignment="Top" DataContext="{Binding SelectedItem, ElementName=listView1, Mode=TwoWay }" />
    </WrapPanel>
    <TextBox  Grid.Column="1" Grid.Row="2" Grid.ColumnSpan="2" Grid.RowSpan="3"  Margin="10,0,0,0"
               Text="{Binding SelectedItem, ElementName=listView1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged }"  TextWrapping="Wrap" AcceptsReturn="True"  >

    </TextBox>

Any ideas.