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DERIVATIVE FINANCIAL INSTRUMENTS Derivative Financial Instruments (Credit Risk) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Derivative [Line Items]          
Letter of Credit Available Commodity Derivatives asset position $ 0.5   $ 0.5   $ 1.2
Commodity Derivative, Net Asset Position, Aggregate Fair Value 2.8   2.8   4.0
Collateral Already Posted, Aggregate Fair Value 25.3   25.3   28.9
Additional Collateral, Aggregate Fair Value (1.2)   (1.2)   (4.8)
Derivative, Net Liability Position, Aggregate Fair Value 24.1   24.1   33.7
Cash collateral to request from interest rate derivative counterparty 0.0   0.0   53.5
Interest Rate Derivative, net asset position, Aggregate Fair Value 0.0   0.0   53.5
SCEG          
Derivative [Line Items]          
Collateral Already Posted, Aggregate Fair Value 10.9   10.9   10.1
Additional Collateral, Aggregate Fair Value (0.6)   (0.6)   (4.6)
Derivative, Net Liability Position, Aggregate Fair Value 10.3   10.3   14.7
Cash collateral to request from interest rate derivative counterparty 0.0   0.0   53.5
Interest Rate Derivative, net asset position, Aggregate Fair Value 0.0   0.0   $ 53.5
Not Designated as Hedging Instrument [Member] | Interest Rate Contract          
Derivative [Line Items]          
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net 0.0 $ (35.0) 65.0 $ (24.0)  
Designated as Hedging Instrument [Member] | Interest Rate Contract | Cash Flow Hedging [Member]          
Derivative [Line Items]          
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net   (1.0) 2.0 (1.0)  
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Contract          
Derivative [Line Items]          
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income (1.0)   (1.0)    
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Interest Rate Contract | Cash Flow Hedging [Member]          
Derivative [Line Items]          
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income (1.0) $ 0.0   $ (1.0)  
Other Income [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Contract          
Derivative [Line Items]          
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income $ 0.0   $ 115.0