Internet Data Center (IDC) supports the reliable operations of many important Internet on-line services. As the demand on Internet services and cloud computing keep increasing in recent years, the power usage associated with IDC operations has been uprising significantly. The cyber and physical aspects of IDCs interact with each other, and brings unprecedented challenges in power management. While most existing research focuses on reducing power consumptions of IDCs, this paper studies the problem of minimizing the total electricity cost geared to quality of service constraint as well as the location diversity and time diversity of electricity price under multiple electricity markets. We jointly consider both the cyber and physical management capabilities of IDCs, and exploit both the center-level load balancing, and the server-level power control in a unified scheme. We model the problem as a constrained mixed integer programming based on Generalized Benders Decomposition (GBD) technique. Extensive evaluations based on real-life electricity price data for multiple IDC locations demonstrates the effectiveness of our scheme.