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Acquisition by Duke Energy Corporation (Details) - USD ($)
1 Months Ended 2 Months Ended
Nov. 30, 2016
Oct. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]        
Costs to achieve expenses     $ 7,300,000 $ 5,700,000
North Carolina Utilities Commission | Approved Stipulation and Settlement Agreement September 2016        
Public Utilities, General Disclosures [Line Items]        
One Time Customer Bill Credit     $ 10,000,000  
Increase (Decrease) in Accounts Receivable $ (5,300,000) $ (4,700,000)