Angels have the Phone Box
No, W10 is not impervious to a Ransomware attack. There are several options available to you; one you're already implementing: Keeping a backup offline. Some others to consider:
I have heard that the Malwarebytes Beta Anti-Ransomware may not yet be ready for prime-time, but have not tried it myself, and the evaluations I am referring to are several weeks old. This feature will eventually be rolled into their paid product.
Malwarebytes Antiexploit is another good little program to have installed, protecting your browsers against zero-day exploits.
As always, the best defense is the person between the keyboard and the screen. However, even the safest surfer can be hit by an exploit kit on a compromised site, (which is recently another method of attack for the ransomware), so it's important to be protected with a layered approach.
Hope that helps.