This is the same income you are reporting to California. As your resident state, Idaho taxes all of your income no matter where you made it. California taxes the income you made working in California as a nonresident. The income you earned in California, therefore is the income taxed by both states. Idaho wants to know this information because Idaho will give you a credit for the amount of tax you pay to California on the income you earned there.
Generally speaking, TurboTax will transfer this information over to the Idaho return if you prepare the California nonresident return first. You may want to delete the Idaho return, prepare the California return, and then re-prepare the Idaho resident return for the credit to transfer over. Otherwise, you will have to look up the amount of income you are reporting to California, and the amount of tax you are paying. The amount of tax is not what is reported on the CA W2, but rather the CA withholdings minus CA refund (or in addition to CA additional tax due).