<xs:complexType name="IdentityDocument">
    <xs:sequence>
      <xs:element name="number" type="xs:string" minOccurs="0" nillable="true"/>
      <xs:element name="serial" type="xs:string" minOccurs="0" nillable="true"/>
    </xs:sequence>
  </xs:complexType>

public class IdentityDocument {
    @XmlElementRef(name = "number", type = JAXBElement.class, required = false)
    protected JAXBElement<String> number;
    @XmlElementRef(name = "serial", type = JAXBElement.class, required = false)
    protected JAXBElement<String> serial;
...
}

public class IdentityDocument {
    protected String number;
    protected String serial;
...
}