The overview-detail design pattern, characterized by an overview of multiple items and a detailed view of a selected item, is ubiquitously implemented across software interfaces. Designers often try to account for all users, but ultimately these interfaces settle on a single form. For instance, an overview map may display hotel prices but omit other user-desired attributes. This research instead explores the malleable overview-detail interface, one that end-users can customize to address individual needs. Our content analysis of overview-detail interfaces uncovered three dimensions of variation: content, composition, and layout, enabling us to develop customization techniques along these dimensions. For content, we developed Fluid Attributes, a set of techniques enabling users to show and hide attributes between views and leverage AI to manipulate, reformat, and generate new attributes. For composition and layout, we provided solutions to compose multiple overviews and detail views and transform between various overview and overview-detail layouts. A user study on our techniques implemented in two design probes revealed that participants produced diverse customizations and unique usage patterns, highlighting the need and broad applicability for malleable overview-detail interfaces.