WASHINGTON (AP) -- Fraud and billing errors in the Medicare program cost the government an estimated $11.6 billion last year, a slight improvement over previous years, the agency that runs the program said Friday. The error rate - claims that were medically unnecessary, inadequately documented or improperly coded - was 5.8 percent, down from 6.3 percent the year before, the Centers for Medicare and Medicaid Services said. The error rate was as high as 13.8 percent in 1996. But the CMS report was immediately attacked by a Republican senator who asserted it understates the problem and is "not statistically valid."...