Minecraft wiki entry for World_boundary, associated text: It is impossible to teleport beyond X/Z ±30,000,000 using commands in the vanilla game. Any attempts to teleport further puts the player back to this coordinate. Attempting to use any block placement command results in an error message. Despite this, it is still possible to use structure blocks and the /locate command.
X/Z ±31,999,872 are the maximum coordinates reachable in the Overworld via Nether portal. The only way to teleport beyond these coordinates is to use external tools (such as a world editor).