Dallas-based pipeline operator Energy Transfer Partners LP, which is seeking regulatory approval to build a shale oil pipeline that would cross Iowa, announced today it has agreed to buy Regency Energy Partners LP in a cash-and-stock deal valued at about $18 billion that includes debt, the Dallas Business Journal reported. Falling crude oil prices were the driving force behind Regency’s decision to sell to ETP, Regency CEO Mike Bradley said in a release. The transaction is expected to close in the second quarter of 2015. The combined entity would be the second-largest master limited partnership, a structure that allows entities to pass through much of their earnings to investors without paying federal income tax, according to Bloomberg News.