The state's largest utility says its customers would get a $40 million break on their power bills as part of a proposal to reduce its rates.  Idaho Power says customer rates would drop by an average of 4.78 percent starting June 1 under the annual power cost adjustment plan it submitted to the state Public Utilities Commission this month. The utility reported the details of the proposal in a statement Monday. A typical residential customer is expected to see savings of nearly $3 a month, under the plan. The overall rate reduction would be smaller at $25 million, or 3 percent, if the commission approves two separate proposals from Idaho Power to bump up rates in order to recover costs. The utility has 492,000 residential, business and agricultural customers.