Clear textbox with the onClick attribute and on a mouseOut fill it

 Hi everybody,
 I have an textbox with an default value. When I click in the textbox it clears the text. I have do that with the follow code:
 <asp:TextBox ID="TextBoxDeclaratieMaandag" runat="server" OnClick="this.value=''" Font-Italic="True" ForeColor="#999999">Declaratiebedrag..</asp:TextBox>
My question is: How can I fill it with the default value when an user doesnt fill the textbox after he clicks in in it. So when I click outside the textbox it must check or the textbox has an value. If not fill it with the default value.

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