Berkshire has benefitted from insurance float for a long time, but it's a minor benefit. Float can't be invested in the stock market, it's extremely restricted in it's usage, and it's only been helpful because of the incredibly disciplined way Berkshire runs it's insurance companies. Proof is it never gave Berkshire Hathaway remotely the returns the Partnerships achieved.
Buffett has said it ad nauseum- the anchor around his neck is the size. Beating the market with what $200 billion to invest? That's leagues different from beating it with $10 million.
(Can you tell I just plugged a bunch of random years plus "Berkshire letter" into Google? I didn't find a single one that didn't play up insurance; I'm not leaving any out.)
In 1994, he even laments exiting a position in GEICO in the 1950s as one of his major mistakes!
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.