According to a recent survey,

...hospitals lost $45.9 Billion, or 6.1% of total costs, in uncompensated care in 2012. This includes both bad debt and charity care, but not underpayments by Medicare and Medicaid.

Source: "Hospitals Lose $45.9 Billion in Uncompensated Care in 2012," The Hospitalist, abstract only, July 2014, http://www.the-hospitalist.org/details/article/6315041/Hospitals_Lose_45_9_Billion_in_Uncompensated_Care_in_2012.html