Dung intended his abstract argument frameworks to be used for modeling a particular form of human argumentation, where arguments attack each other and are evaluated following the principle summarized by "The one who has the last word laughs best." However this form does not fit a wide class of arguments, which is arguably more prototypical and common in human argumentation, namely arguments where pros and cons are balanced to choose among alternative options. Here we present a formal model of structured argument which generalizes Dung abstract argumentation frameworks to also handle balancing. Unlike most other models of structured argument, this model does not map structured arguments to abstract arguments. Rather it generalizes abstract argumentation frameworks, allowing them to be simulated using structured arguments. The model can handle cumulative arguments ("accrual") without causing an exponential blowup in the number of arguments and has been fully implemented in Version 4 of the Carneades Argumentation System.