That is why I use bitcoinarmory and a paper wallet. You can also back up to a usb stick and put it in your safe along with your paper wallet. Then create a watch-only wallet, wipe armory's user directory, install the watch only wallet (which is write only) and then you have true security. If you every want to spend from your offline wallet, use the mobile app mycelium to access offline BTC.
RE: Trezor Wallet