Attacks on computer networks have increased significantly in recent days, due in part to the availability of sophisticated tools for launching such attacks as well as thriving underground cyber-crime economy to support it. Over the past several years, researchers in academia and industry used machine learning (ML) techniques to design and implement Intrusion Detection Systems (IDSes) for computer networks. Many of these researchers used datasets collected by various organizations to train ML models for predicting intrusions. In many of the datasets used in such systems, data are imbalanced (i.e., not all classes have equal amount of samples). With unbalanced data, the predictive models developed using ML algorithms may produce unsatisfactory classifiers which would affect accuracy in predicting intrusions. Traditionally, researchers used over-sampling and under-sampling for balancing data in datasets to overcome this problem. In this work, in addition to over-sampling, we also use a synthetic data generation method, called Conditional Generative Adversarial Network (CTGAN), to balance data and study their effect on various ML classifiers. To the best of our knowledge, no one else has used CTGAN to generate synthetic samples to balance intrusion detection datasets. Based on extensive experiments using a widely used dataset NSL-KDD, we found that training ML models on dataset balanced with synthetic samples generated by CTGAN increased prediction accuracy by up to 8%, compared to training the same ML models over unbalanced data. Our experiments also show that the accuracy of some ML models trained over data balanced with random over-sampling decline compared to the same ML models trained over unbalanced data.