Census Improving Data and Enhanced Accuracy Act or the Census IDEA Act This bill prohibits the Department of Commerce, in the 2020 decennial census of population and each decennial census thereafter, from (1) implementing any major operational design element that has not been researched, studied, and tested for a period of at least three years before the decennial census date; or (2) including on the questionnaire for the census any subject, type of information, or question that was not submitted to Congress. The Government Accountability Office shall determine and report to Congress on whether the subjects, types of information, and questions to be included in that decennial census have been adequately researched, studied, and tested to the same degree as in previous decennial censuses.