If you're really talking about having that much Bitcoin, I would suggest having a look at a hardware wallet.  These are essentially separate mini-computers which cannot be infected with malware as they do not transfer private keys with other computers to transfer.  I don't have that much of an understanding since I don't have enough Bitcoin to bother, but you could read up on TREZOR, which I know is a pretty prominent one.  If you're not too concerned about malware you could have an offline wallet which would be 99.999% safe anyway.  Definitely wouldn't keep them online though because once you're hacked you're screwed.