A system includes a roof and an airbag supported by the roof. The airbag is inflatable away from the roof to an inflated position. The airbag includes an apex adjacent to the roof. The airbag includes a first leg inflatable away from the apex to an end, and a second leg inflatable away from the apex to an end spaced from the end of the first leg. The airbag includes a bottom portion spaced from the apex and inflatable to the inflated position. The bottom portion extends from the end of the first leg to the end of the second leg.