1. A method comprising: obtaining a collection of training documents, the training documents including a group of positive documents and a group of negative documents, wherein the positive documents are training documents identified as being longform documents and the negative documents are training documents identified as not being longform documents; extracting a plurality of features from the training documents, wherein the plurality of features are associated with a plurality of different feature types that represent lexical or textual content of the training documents that are indicative of a document's writing style; generating a longform document classifier trained using feature instances extracted from the training documents, wherein the generated longform document classifier is trained such that input documents are classified as being longform documents or classified as not being longform documents; applying the longform document classifier to a corpus of documents; annotating an information retrieval index with an output classification for each document of the corpus of documents; and using the annotated index to provide search results identifying longform documents in response to a search query.