It's actually the reverse. Buffett requires his insurance companies write profitable policies, so that interest earned on the float is gravy. When one suffers through an inevitable bad year (Gen Re a recent example) float interest is a greater margin of safety.

Most insurers write unprofitable policies and use float interest to cover those losses and make a profit. When they have a bad year, they often lose horrendous amounts and can be forced to raise more funding at unattractive rates just to stay in business.

And the value of float seems to be over-rated on this site. Float almost never can be invested in the stock market or other high yield aggressive investments. Because it represents policyholder's premium dollars it has to be invested very conservatively, usually in safe fixed income investments. The way Buffett uses it adds a nice bump to earnings, but it's far from a key to his success. He never owned any insurance companies when he had his highest return decades.