2020 Census Improving Data and Enhanced Accuracy Act or the 2020 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 that has not been researched, studied, and tested for a period of not less than three years before the date on which the applicable census occurs; 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 must determine that the subjects, types of information, and questions to be included in a decennial census have been adequately researched, studied, and tested to the same degree as in previous decennial censuses.