820 ILCS 305/2
An employer in this State, who does not come within the classes enumerated by Section 3 of this Act, may elect to provide and pay compensation for accidental injuries sustained by himself or any employee, arising out of and in the course of the employment according to the provisions of this Act, and thereby relieve himself from any liability for the recovery of damages, except as herein provided. The State of Illinois hereby elects to provide and pay compensation according to the provisions of this Act.
Any such employer who may have once elected, may elect not to provide and pay the compensation herein provided for accidents resulting in either injury or death and occurring after the expiration of any such calendar year by filing notice of such election with the Commission at least 60 days prior to the expiration of any such calendar year, and by posting such notice at a conspicuous place in the plant, shop, office, room or place where such employee is employed, or by personal service, in written or printed form, upon such employees, at least 60 days prior to the expiration of any such calendar year.
Every employer within the provisions of this Act who has elected to provide and pay compensation according to the provisions of this Act by insuring his liability to pay compensation under this Act, as above provided, shall be bound thereby as to all his employees until the date of expiration or cancellation of such policy of insurance, or any renewal thereof.
However, any employee may withdraw from the operation of this Act, except those under Section 3, upon filing a written notice of withdrawal at least 10 days prior to January 1st of any year with the Commission, whose duty it shall be to immediately notify such employer by registered mail, and, until such notice to the contrary is given to such employer, the measure of liability of such employer shall be determined according to the compensation provisions of this Act.
820 ILCS 305/2