DERIVATIVE FINANCIAL INSTRUMENTS Derivative Financial Instruments (Credit Risk) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative [Line Items]
Letter of Credit Available Commodity Derivatives asset position	$ 0.9		$ 1.2
Commodity Derivative, Net Asset Position, Aggregate Fair Value	3.2		4.0
Collateral Already Posted, Aggregate Fair Value	27.1		28.9
Additional Collateral, Aggregate Fair Value	(0.9)		(4.8)
Derivative, Net Liability Position, Aggregate Fair Value	26.2		33.7
Cash collateral to request from interest rate derivative counterparty	0.0		53.5
Interest Rate Derivative, net asset position, Aggregate Fair Value	0.0		53.5
SCEG
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	10.9		10.1
Additional Collateral, Aggregate Fair Value	(0.1)		(4.6)
Derivative, Net Liability Position, Aggregate Fair Value	10.8		14.7
Cash collateral to request from interest rate derivative counterparty	0.0		53.5
Interest Rate Derivative, net asset position, Aggregate Fair Value	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	65.0	$ 11.0
Not Designated as Hedging Instrument [Member] | Interest Rate Contract | SCEG
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net	65.0	11.0
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	2.0	0.0
Designated as Hedging Instrument [Member] | Interest Rate Contract | Cash Flow Hedging [Member] | SCEG
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net	65.0	11.0
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	0.0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Contract | SCEG
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income	0.0	(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	0.0	(1.0)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Interest Rate Contract | Cash Flow Hedging [Member] | SCEG
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income		(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	115.0
Other Income [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Contract | SCEG
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net		(24.0)
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income		$ (1.0)
Other Income [Member] | Designated as Hedging Instrument [Member] | Interest Rate Contract | Cash Flow Hedging [Member] | SCEG
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income	$ 115.0