Are you sure you want that? If you watch a fifteen minute video, the system might shutdown at 2/3.
Usually 'idle' is based on last keystroke or mouse button or user interaction.
You can't really use CPU utilization because that varies even when 'idle'.
This is a complex issue.
2 of Seagate Barracuda XT ST32000641AS (2TB ea.);
1 of Seagate Barracuda Green ST2000DL003 (2TB);
1 of Hitachi Deskstar HDS722020ALA330 (2TB);
2 of Seagate Desktop ST4000DM000-1F2168 (4TB)
Corsair AX850 Gold
Cooler Master HAF 932 Advanced
ThermalTake Silent 1156
Event Studio Precision 6 powered audio monitors;
Boston Acoustics CS Sub 10 Powered Subwoofer;
NI Kore controller;
NI Maschine controller;
M-Audio Axiom 61 keyboard controller; expression pedal; sustain pedal;
... and tons of audio software ...
I also keep two USB 3 thumb drives (A: and B:) attached with boot recovery and security stuff that I can boot into from BIOS in case of emergency
Instead of the utility, you can also use the Power Settings and either use one of the three profiles, or make up your own. I set when on battery, to shut down or sleep after 10 min's, shut screen off when lid is closed.