In a dispute involving the rightful allocation of asbestos-related losses under thirty-year-old excess insurance policies, and to decide which of two companies, a historical manufacturer of asbestos-containing products and its insurer, will bear costs associated with a staggering number of asbestos claims, the district court's judgment that the policy is ambiguous is reversed where the phrase 'arising out of' asbestos, when used in a Pennsylvania insurance exclusion, unambiguously requires 'but for' causation.