I've had battery usage problem on my Galaxy6 for months which started the day I upgraded to nougat. Highest usage is Device idle followed by Android System. I do nothing with the phone. The factory reset didn't help, nor cache wipes. Safe mode and MAX power saving mode turn off all the extra apps, and that does not help. I used package disabler and disabled lots of stuff and that doesn't help. I completely discharged and recharged the battery. I've "reset" the battery (procedure described on another thread in this forum). Same result: about 3 times the battery use compared to MM, no matter whether I use the phone or not (if I use it, it burns about 2-3 times the power for the same tasks).
I also went to the Sprint store to ask to go back to MM. They couldn't or wouldn't, but they did reflash nougat for me which was nice. But it didn't help. In short, nothing helps, there's no solution. But I might try a different launcher (mentioned above). Just noticed "developer mode" was on, although none of the developer mode features were on. So I just turned that off. Turning off developer didn't seem to help. Also my launcher is touchwiz which must be Samsung, so I left it alone.