If you are enrolled in SURS, you are automatically covered by its disability benefits after 2 years. Through this, you will receive 50% of your pay. The state of Illinois pays all premiums, and benefits begin 60 days after SURS approves them.
In addition to SURS coverage, you can enroll in University insurance. The cost of this is determined based on your age and salary. This plan pays an additional 16.67% beyond what SURS pays, so your total income will be 66.67% of what it was when you were working. You might also receive up to 20% more based on the type of illness or injury you have.
More information on both plans here: https://www.hr.uillinois.edu/benefits/lifedisability/ltd and here: https://www.hr.uillinois.edu/benefits/lifedisability/ltd/faqs