According to a recent study...

... the percentage of workers covered by health plans, who were enrolled in plans that were self-insured by their employers, increased from 58.2% to 60% from 2013 to 2015.

Source: "Self-Insured Health Plans: Recent Trends by Firm Size,1996‒2015, p.2," Employee Benefti Research Institute Notes, July 2016, https://www.ebri.org/pdf/notespdf/EBRI_Notes_07-no7-July16.Self-Ins.pdf