Their second album, "Days of Future Passed", which was released in 1967, was a fusion of rock with classical music which established the band as pioneers in the development of art rock and progressive rock. It has been described as a "landmark" and "one of the first successful concept albums". The group toured extensively through the early 1970s, then took an extended hiatus from 1974 until 1977. Founder Mike Pinder left the group a year after they re-formed and was replaced by Swiss keyboardist Patrick Moraz in 1978. In the following decade they took on a more synth-pop sound and produced "The Other Side of Life" in 1986, which made them the first act to earn each of its first three top 10 singles in the United States in a different decade. Health troubles led to a diminished role for founder Ray Thomas throughout the 1980s, though his musical contributions rebounded after Moraz departed in 1991. Thomas retired from the band in 2002.