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DERIVATIVE FINANCIAL INSTRUMENTS Derivative Financial Instruments (Credit Risk) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Derivative [Line Items]    
Letter of Credit Available Commodity Derivatives asset position $ 1.3 $ 1.5
Commodity Derivative, Net Asset Position, Aggregate Fair Value 5.0 9.0
Collateral Already Posted, Aggregate Fair Value 31.0 29.2
Additional Collateral, Aggregate Fair Value 36.1 21.1
Derivative, Net Liability Position, Aggregate Fair Value 67.1 50.3
Cash collateral to request from interest rate derivative counterparty 49.0 62.9
Interest Rate Derivative, net asset position, Aggregate Fair Value 49.0 62.9
SCEG    
Derivative [Line Items]    
Collateral Already Posted, Aggregate Fair Value 10.3 9.2
Additional Collateral, Aggregate Fair Value 37.4 21.1
Derivative, Net Liability Position, Aggregate Fair Value 47.7 30.3
Cash collateral to request from interest rate derivative counterparty 49.0 62.0
Interest Rate Derivative, net asset position, Aggregate Fair Value $ 49.0 $ 62.0