Pipeline operator Energy Transfer Partners LP said it would buy affiliate Regency Energy Partners LP in a deal valued at about $18 billion, including debt.

Regency shareholders will receive 0.4066 of an Energy Transfer unit and 32 cents in cash for each Regency unit they own.

The offer works out to $26.89 per Regency unit based on Energy Transfer's Friday close, a 13 percent premium to Regency's last closing price.