Hello you-i,
You are right it is not explicitly given. However we can infer what the specification would be. First of all the output of the part will clip at 10.5Vrms with 18V rails. This is +- 14.847 V peak. So this is where it will clip. The differential output voltage swing specification is +-14.14V typical with 7.071 Vrms input level. So there is no reason to go much beyond 7Vrms to keep the part from clipping.
Then for the reason of protecting the part from damage, which I think is what you are asking about. You do not want to exceed the voltage rails of the part. So that would be +-18V peak.