Div with margin-left and width:100% overflowing on the right side

I have 2 nested div's that should be 100% wide. Unfortunately the inner div with the Textbox overflows and is actually larger than the outer div. It has a left margin and overflows by about the size of the margin.

How can I fix that?

<div style="width:100%;">
    <div style="margin-left:45px; width:100%;">
    <asp:TextBox ID="txtTitle" runat="server" Width="100%"></asp:TextBox><br />
    </div>
</div>

If I don't do the 100%, then the textbox is not 100% wide.