The Expected Shortfall (ES) is one of the most important risk measures widely applied in the field of finance, insurance, statistics, and risk management. In light of recent results characterizing ES in the context of financial regulation and statistics, we examine the implication of ES in insurance and actuarial science. In this paper, we study a reinsurance contract design problem focusing on the risk measure used by an overseer. One of our major results is that we characterize a mixture of the mean and ES as the risk measure of the overseer, where the optimal contracts are within the common set of ceded loss functions with a deductible form. Characterization results of other classes of risk measures including the mean and the distortion risk measures are demonstrated as the optimal set of ceded loss functions changes. Extension to the case with multiple reinsurers and alternative explanations of the optimal contract condition from the perspective of the insurer are also discussed.