There is no universally accepted format for citing government documents, anymore than there is for any other source of information. The general guidelines given below should always be used in conjunction with one of the standard style manuals.
Why bother citing your sources, you ask?
"Plagiarism is defined as presenting someone else’s work, including the work of other students, as one’s own. Any ideas or materials taken from another source for either written or oral use must be fully acknowledged, unless the information is common knowledge. What is considered “common knowledge” may differ from course to course.
A student must not adopt or reproduce ideas, opinions, theories, formulas, graphics, or pictures of another person without acknowledgment.
(quoted from Code of Student Rights, Responsibilities, & Conduct, Student Responsibilities, Uphold and maintain academic and professional honesty and integrity, By action of the University Faculty Council (February 24, 2015) and the Trustees of Indiana University (March 1, 2015).)