Idris Elba is not a superstar actor. I'd say he has a good chance of being one but he's not a marquee name by a long shot -- he's appeared in some big films but usually in supporting roles.
Matthew McConaughey is more of one lately with Interstellar but he's still a guy who does a lot of independent films over big budget affairs. 'Dallas Buyers Club' cost five million to make and made 27 million domestically for example (which is a big success given how cheap it was made for but it wasn't exactly a blockbuster based on his star power).