1. A method of detecting an atrial tachyarrhythmia in a medical device, comprising: sensing a first cardiac electrical signal from a first sensing electrode vector; identifying R-waves in the first cardiac electrical signal attendant ventricular depolarizations; determining RR intervals between successive R-waves in the first cardiac electrical signal; determining classification factors from the R-waves identified over a first predetermined time period by determining at least a first classification factor correlated to variability of the RR intervals and a second classification factor indicating a presence of a ventricular tachyarrhythmia; classifying the first predetermined time period as one of unclassified, atrial tachyarrhythmia and non-atrial tachyarrhythmia by comparing the determined classification factors to classification criteria, wherein the first predetermined time period is classified as unclassified in response to the second classification factor indicating the presence of a ventricular tachyarrhythmia; detecting an atrial tachyarrhythmia episode in response to the first time period being classified as unclassified and at least one preceding time period and at least one subsequent time period being classified as atrial tachyarrhythmia; and in response to detecting the atrial tachyarrhythmia episode, storing episode data determined from at least the first cardiac electrical signal in a memory of the medical device for transmission to an external device.