Black Hills falls on deal to acquire NorthWestern Energy for $3.6 billion

** Shares of utility company Black Hills BKH.N down 3.6% at $58.20 premarket

** Co says it would buy peer NorthWestern Energy Group NWE.O for $3.6 billion

** Under the agreement, NWE shareholders will receive 0.98 shares of BKH for each held

** The all-stock deal will create a regional regulated natural gas utility company with a pro forma market capitalization of roughly $7.8 billion and a combined enterprise value of $15.4 billion

** The per-share value of about $59 implies a premium of 7.66% to NorthWestern's last close, according to Reuters calculation

** Transaction expected to close within 12 to 15 months

** As of last close, BKH shares up 3.1% YTD