Cassidy, Ivanishin and Vagner spent 196 days in orbit, having arrived at the station on April 9.
Cassidy, returning from his third space mission, has now spent a total of 378 days in space, the fifth highest among U.S.
While serving as the station's commander, Cassidy welcomed SpaceX Demo-2 crew Robert Behnken and Douglas Hurley, the first NASA astronauts to launch to the space station on an American spacecraft from American soil since the retirement of the space shuttle fleet in 2011.