Large Supplier Overpayment

We received notice that our local electric company had been over charging us for years so we have been issued a 20K credit to our account. I am a little puzzled about the best way to handle this transaction: - I am inclined to book the entire credit to the expense account in the current period, despite the fact that the over payment includes reimbursement for prior fiscal years. - My thinking is that this will then greatly reduce our YTD expense in this GL, but would be balancing out the overstatement of that account in prior years? Does this seem correct, or is there a better approach? My other thought is to park the "credit" in prepaid and then chip away at it as we receive new invoices from the electric co monthly. Thanks!