Short answer: Illinois does not set its own minimum employer contribution by law. As in most states, the carrier sets the minimum, commonly around 25% to 50% of the employee-only premium, and it is waived during the year-end guaranteed-issue window.
There is no Illinois statute requiring employers to pay a specific share of the premium. In practice, carriers require an employer to pay a minimum portion of the premium to issue a small-group plan, often in the range of 25% to 50% of the employee-only premium, and they typically require around 70% participation. Both are waived if the group enrolls during the annual November 15 to December 15 guaranteed-issue window for a January 1 effective date.
One related figure is often confused with a state rule: to claim the federal Small Business Health Care Tax Credit, an employer must pay at least 50% of employee-only premiums. That is a condition for the tax credit, not a requirement to offer coverage.