Warren Buffett’s Berkshire Hathaway had its credit grade lowered one level by Standard & Poor’s after the ratings firm revised its criteria for evaluating insurance holding companies.

Berkshire was cut to AA from AA+. Its outlook on all of Berkshire’s ratings is negative. So far, the downgrades haven’t hurt its standing in the bond markets.

The company held the highest credit rating from S&P, Fitch Ratings and Moody’s as recently as 2009.