According to a recent analysis, in 2013 (the most recent year that data is available):

• 37.6% of private-sector employers self-insured at least one of their health plans, up from 26.5% in 1999
• 58.2% of workers with health coverage were in self-insured health plans, up from 40.9% in 1998
• 83.9% of employers with 500 or more employees self-insured at least one health plan, up from 66.2% in 1999
• 64.6% of firms with 50 or more employees offered at least one self-insured plan, compared with 13.2% among firms with fewer than 50 employees
• 13.2% of workers in firms with fewer than 100 employees were in self-insured plans, down from 20.7% in 1997

Source: "Self-Insured Health Plans: State Variation and Recent Trends by Firm Size, 1996–2013," Employee Benefit Research Institute (EBRI), June 2015, http://www.ebri.org/pdf/notespdf/EBRI_Notes_06_June15_SI-AutoIRAs.pdf