In Xanadu did Kubla Khan / A stately pleasure-dome decree. Those lines are from the poem "Kubla Khan" (published in 1816) by Samuel Taylor Coleridge. Coleridge's fantastic description of an exotic utopia fired public imagination and ultimately contributed to the transition of "Xanadu" from a name to a generalized term for an idyllic place. The Xanadu in the poem was inspired by Shang-tu, the summer residence of Mongolian general and statesman Kublai Khan (grandson of Genghis Khan). You might also recognize "Xanadu" as the name of the fantastic estate in Orson Welles's 1941 film Citizen Kane.
1919, in the meaning defined above
Xanadu, locality in Kubla Khan (1798), poem by Samuel T. Coleridge