The EPA set variable CO2 goals for power plant emissions from state to state, and the state with the largest target reduction percent is Washington State.
Vox's Brad Plumer gives a nice overview of the EPA CO2 rule and how it is supposed to work HERE.
The biggest reason Washington State's reduction is so high even though are CO2 per megawatt hour is the lowest in the country is the one coal power plant in the state will be shutting down. That alone will get Washington close to the EPA target. The remaining reductions will require efficiency schemes or CO2 reductions elsewhere or developing more wind, hydro or nuclear.
Overall the variable target approach by the EPA is appropriate. It recognizes the big differences in energy sources across the country and differences in CO2 reduction opportunities and takes into account actions that are already in the works such as Washington State's closing of the coal power plant in Centralia or South Carolina's building of a large nuclear power plant.