The electrical emanations from the typewriter can be used to detect the characters typed regardless of whether or not an electronic display is used, and TEMPEST

The electrical emanations from the typewriter can be used to detect the characters typed regardless of whether or not an electronic display is used, and TEMPEST, which is a method of securing electronic devices, will protect the typewriter's emanations from outside detection, and, as mentioned by Daniel_Newby, electric typewriters are vulnerable to power line reading, but if the system is entirely disconnected from any external power source, as TEMPEST dictates, and the entire facility is held to TEMPEST standards, then the typewriter will be secured against external eavesdropping. Any attempts at picking up audio|electronic emanations will be foiled by traditional TEMPEST type protection. Typewriter ribbons can be dealt with by old fashioned incineration. The only way to acquire a signal would be by infiltration of the facility housing the targeted systems.