*After seven years of cultivating the careers of Island Def Jam superstars like Rihanna, Kanye West, and Justin Bieber, Island Def Jam Chairman LA Reid resigned this morning, reports EW.com.

In a letter to his staff, Reid explains that he’s “always thrived on growth and the next great challenge, and I look forward with much enthusiasm to what the future holds.”

The move comes in advance of an expected announcement this week that Reid will join Simon Cowell as a judge on the new singing competition series “The X Factor.”

Below is LA’s full letter to his staff:

To my Island Def Jam family:

After much consideration, I have decided to leave my position as Chairman of the Island Def Jam Music Group. I have always thrived on growth and the next great challenge, and I look forward with much enthusiasm to what the future holds. I am extremely proud of our beautiful roster and all we have accomplished in my seven years with IDJ.  We continue to have incredible success together with today’s most phenomenal superstars – Rihanna, Justin Bieber, Mariah Carey, Kanye West, Bon Jovi, Jennifer Lopez, Ne-Yo, Rick Ross, The Killers, The-Dream, Chrisette Michele, Jeezy and Ludacris to name a few. I want to thank all of you for your amazing contributions.

With Warm Regards –

LA