A Dash 8 partially exited the paved surface when the flight crew lost control of the aircraft while taxiing on an unplowed taxiway.

Narrative

Passenger boarding; deicing and engine start completed normally. We taxied off the ramp and proceeded northeast on Taxiway Alpha toward the runway. [We were] approximately 150 yards after leaving ramp; while traveling approximately 5 KTS; the aircraft began to slide to the left. All attempts to arrest the slide with nosewheel steering; wheel brakes and differential thrust were unsuccessful. In a matter of a few seconds the aircraft slid to a gentle stop at the left edge of Taxiway Alpha with the left main landing gear off the paved surface approximately eighteen inches; on solid hard soil and embedded in a snow bank about twelve inches deep. The nose wheels were straddling the double-yellow taxiway edge stripes on asphalt.We set the parking brake immediately and notified ATC and Company Operations. No attempts were made to move the aircraft any further under its own power. The passengers were deplaned and transported to the terminal. After pinning the main landing gear and locking the nose gear airport personnel cleared snow from around left main and nose landing gear; and then towed the aircraft to the ramp. No passengers or crewmembers were injured; and no aircraft damage was apparent upon initial inspection.It would have been helpful to have a NOTAM on ATIS or pilot reports indicating taxiway conditions. It appeared that airport personnel had only partially plowed the airport and the only access to our runway was along Taxiway Alpha; which had not been cleared.