1. A method comprising: outputting, by one or more computing devices and for display, a first interface area in which an editable document is displayed and a second interface area outside of the first interface area; in response to a paste command, inserting copied content into the editable document; identifying a plurality of candidate sources for the copied content, each candidate source being a distinct source of the copied content, at least some of the candidate sources being identified based on documents concurrently open in a third interface area outside of the first interface area and the second interface area; outputting, in the second interface area, a plurality of interface elements, wherein each respective interface element corresponds to a respective candidate source for the copied content; receiving, from a user, a selection of one of the interface elements; and generating, by the one or more computing devices, citation information in the editable document based on the candidate source associated with the selected one of the interface elements.