Even though Warren Buffett'sBerkshire Hathaway ( BRK.A), ( BRK.B) hasn't filed its own 13F yet (the firm held a stock portfolio worth $74 billion at last count), it's certainly been present on other investment managers' filings. In the first quarter of 2013, funds picked up 5.92 million shares of Berkshire, adding onto a 77.49 million-share position in the conglomerate. While most investors know Berkshire Hathaway for the group of disparate businesses it owns, including NetJets, Geico, See's Candy and Lubrizol, it's probably most accurate to call Berkshire Hathaway an insurance company. Despite the many pitfalls that insurers have faced in the last several years, Berkshire has managed to come out ahead, managing underwriting risks adequately and affording themselves a hefty profitability cushion. When interest rates start to come back around, the firm's ability to earn should increase materially. In the meantime, the firm's huge stock portfolio should continue to get buoyed by this rally, and Berkshire should benefit more than most firms. While Warren Buffett's succession plan adds some unknown risks to Berkshire over the longer-term, investors shouldn't discount the long-term performance that the firm has managed to generate -- or his team's ability to replicate the approach in his absence. To see these stocks in action, check out the Q1 2013 Institutional Buys portfolio on Stockpickr. -- Written by Jonas Elmerraji in Baltimore.