The ever-increasing competitiveness in the academic publishing market incentivizes journal editors to pursue higher impact factors. This translates into journals becoming more selective, and, ultimately, into higher publication standards. However, the fixation on higher impact factors leads some journals to artificially boost impact factors through the coordinated effort of a "citation cartel" of journals in addition to self-citations. "Citation cartel" behavior has become increasingly common in recent years, with several instances of cartels being reported. Here, we propose an algorithm---named CIDRE---to detect anomalous groups of journals that exchange citations at excessively high rates when compared against a null model that accounts for scientific communities and journal size. CIDRE detects more than half of the journals suspended by Thomson Reuters due to cartel-like behavior in the year of suspension or in advance. Furthermore, CIDRE detects a large number of additional anomalous groups, which reveal a variety of mechanisms that may help to detect citation cartels at their onset. We describe a number of such examples in detail and discuss the implications of our findings with regard to the current academic climate.