Berkshire Hathaway generates income from Operating Earnings as well from Float.
The book value metric that Buffett uses as proxy for intrinsic value takes both the earnings into account. Focusing only on the operating income part of the metric ignores the other half.
This comprehensive income is detailed in the "Consolidated Statement of Comprehensive Income" on page 4 of 31Mar2012 10Q
This explains why the equity has increased from 168 to 180 Billion USD in the last quarter. (Page 2 of the 10Q)
Anecdotally, we know that the equity portion of the float is about 75 Billion. This 75 Billion portfolio has done well with the rest of the market in the
first quarter of 2012.
(click to enlarge)
Berkshire Book Value is close to the buy back threshold of 10% of Book value
The Price to Book Math is below
Market Cap = 201 Billion
Shareholder Equity (Book Value) = 180 Billion
Price/Book = 1.11 = 11% over book value.
Berkshire will buy back stock if @ 10 percent over book value. We are almost there!