Wordpress - get post author id outside loop

The easiest way would be using get_post_field():

$post_author_id = get_post_field( 'post_author', $post_id );

For more details on this issue: have a look at this StackOverflow answer.

You can use the following:

/**
 * Gets the author of the specified post. Can also be used inside the loop
 * to get the ID of the author of the current post, by not passing a post ID.
 * Outside the loop you must pass a post ID.
 *
 * @param int $post_id ID of post
 * @return int ID of post author
*/
function wpse119881_get_author( $post_id = 0 ){
     $post = get_post( $post_id );
     return $post->post_author;
}