"Berkshire’s debt has benefitted from being highly rated, enjoying a AAA rating..."
"Berkshire’s more anomalous cost of leverage, however, is due to its insurance float. Collecting insurance premia up front and later paying a diversified set of claims is like taking a “loan.” Table 3 shows that the estimated average annual cost of Berkshire’s insurance float is only 2.2%, more than 3 percentage points below the average T-bill rate. Hence, Buffett’s low-cost insurance and reinsurance business have given him a significant advantage in terms of unique access to cheap, term leverage. We estimate that 36% of Berkshire’s liabilities consist of insurance float on average." http://www.businessinsider.com/warren-buffetts-investing-strategy-2013-12