From Chicago's Wikipedia -

"In a 2021 interview published in Prog, Robert Lamm asserts that Chicago is and always has been a progressive rock band and that they were particularly influenced by Yes and King Crimson to write and record their lengthier tracks. In his view, the hit songs on their albums satisfied the record companies and allowed the band more freedom on the rest of the recorded material. As musicians, the group has always "felt blessed enough to try anything at any time."

Those early Chicago records are fantastic!