 Shares of Great Western Bancorp Inc. GWB, 1.85 soared 11.9 in premarket trading Thursday, after the South Dakota-based bank announced an agreement to be acquired by Montana-based First Interstate BancSystem Inc. FIBK, 0.79 in an all-stock deal valued at 2.0 billion. Under terms of the deal, Great Western shareholders will receive 0.8425 First Interstate shares for each Great Western share they own. 