Why is rihlata in 106:2 mansoob?

> eelafihim rihlata ash-shita'i was-sayf - 106:2 > > (their accustomed security in the caravan of winter and summer) 106:2 has the word رِحۡلَةَ (rihlata - caravan/journey) with the مَنصُوبُ (mansoob - accusative) case. But I don't understand why. It's not the object of any verb. It's not being used as an adverb of manner. None of the sisters of إِنَّ (inna) are anywhere in sight. So what is this particular use of the مَنصُوبُ (mansoob) case?