Ah missed that analog read part, you want to read that full 18v spectrum? And don't trust your meter, dc and ac voltage reading aren't perfect especially if both are present at the same time, it could be just pulsating dc not ac or a more complex waveform, you really need to oscope it to know what you are dealing with, you could be trying to solve a nonexistent problem, either way a voltage dividerlike he suggested would be fine, and if you are unsure a 5.1zener used correctly will ensure the voltage after the divider is between 0 and +5v
Basically the zener cathode (+ or banded end) will go to the input, cathode to ground and you will be protected
This would be after your voltage divider