Living in WI but employed in CA. Had established tax residency in CA before moving to WI in the middle of the year. Which state tax returns do I file - CA, WI, or both?

I have been living in Madison, Wisconsin since September 2017, but my current employer is based in California. Prior to moving to Madison, I had established tax residency in California after spending six years there in graduate school. In this situation, which states do I need to file my tax returns for, Wisconsin, California, or both?