Relational database systems are increasingly used to manage XML documents, especially for data-centric XML. In this paper we present a new approach to efficiently manage document-centric XML data based on a generic relational mapping. Such a generic XML storage is especially useful in data integration systems to manage highly diverse XML documents. We focus on efficient insert operations, support of streamed data and fast retrieval of document fragments. Therefore we introduce a new numbering scheme called DLN (Dynamic Level Numbering) and several variants of it. A performance evaluation based on a prototypical implementation demonstrates the high efficiency of DLN.