Reuters: Congressional Democrats on Thursday urged big oil companies to invest more of their record profits into boosting U.S. oil production and developing renewable energy instead of buying back their own stock. Exxon Mobil, which reported on Thursday the biggest quarterly corporate profit in history at $11.7 billion (5.9 billion pounds), has bought back $16 billion of its stock in the first half of 2008. Other oil companies have also spent billions of dollars buying their stock, earning the …