The Cougars (9-4) used a 20-2 run late in the first half to blow open a close game.
Point guard Mike Ladd scored 17 points and grabbed nine rebounds to lead the Cougars, who have won four in a row and six of their last seven games.
Brock Motum added 17 points and seven rebounds for Washington State, which trailed 8-6 midway through the first half before Ladd scored seven points and Motum five during the run which put the Cougars on top 26-10 with 4:37 left in the half.
Chris Hansen's 12 points led Idaho State (1-10), which lost its fifth consecutive game. The Bengals are 0-3 against Pac-12 teams this season with previous losses to Utah and Oregon.
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