For works published after 1977, the copyright lasts for the life of the author plus 70 years. However, if the work is a work for hire (that is, the work is done in the course of employment or has been specifically commissioned) or is published anonymously or under a pseudonym, the copyright lasts between 95 and 120 years, depending on the date the work is published.
All works published in the United States before 1923 are in the public domain. Works published after 1922, but before 1978 are protected for 95 years from the date of publication. If the work was created, but not published, before 1978, the copyright lasts for the life of the author plus 70 years. However, even if the author died over 70 years ago, the copyright in an unpublished work lasts until December 31, 2002. And if such a work is published before December 31, 2002, the copyright will last until December 31, 2047.
(Via Stanford University 2010)