Here are the top causes of why a GPU would run at full clocks at idle...
#1 You're running multiple monitors.
Fix: Enable mutil display power saver using nvidia inspector.#2 Your machine has a virus.
Fix: Scan computer using malwarebytes.#3 One or more programs running in the background are using your GPU.
Fix: Perform a clean boot of windows.#4 Power management mode inside the nvidia control panel is set to prefer maximum performance.
Fix: Set power management mode to adaptive then reboot your computer.#5 Running a resolution of 2560x1440 @ 144Hz.
Fix: Change refresh rate to 120Hz.#6 K-Boost is enabled inside Precision X.
Fix: Disable K-Boost.#7 Shadowplay is enabled inside geforce experience.
Fix: Disable shadowplay inside geforce experience OR completely uninstall geforce experience from your machine (highly recommended)