A CL-65 slid off the hard surface while attempting a 180 degree turn on an icy taxiway.

Narrative

Tower cleared us for a visual approach and stated the braking action was good on the runway and taxiways. We were six miles in trail of an Airbus which was also cleared to land on the same runway. The Airbus landed and exited the runway. The Airbus reported that the braking action on the taxiway was fair. As we touched down; the Tower asked us to give them a braking action report on the high speed taxiway. I elected to taxi down to the next taxiway; which allowed me more stopping room due to the report I just heard. As I exited I noticed that the braking action was not good. I also reported braking action as fair to the Tower. It wasn't until the aircraft made the 180 degree turn on the taxiway that the aircraft began to slide. As the aircraft was sliding; I held the brakes; attempted to turn the nose wheel with the tiller and even tried differential power; with power on the left engine. The taxiway surface had been plowed; but there was still snow on the taxiway. The First Officer called out that we had left the taxiway surface. I immediately reduced the power on the left engine and let the aircraft come to a stop. I let the Tower know that we were in the grass off the taxiway. There was no abrupt stopping or braking during this incident. The aircraft was not subjected to any impact. I did not evacuate the aircraft and the passengers were transported to the terminal via a bus.