DERIVATIVE FINANCIAL INSTRUMENTS On Income Statement (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	$ 1.0	$ 1.0	$ 3.0	$ 0.0	$ (3.0)	$ (2.0)	$ 5.0	$ (5.0)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ (3.0)	$ (2.0)	$ (4.0)	$ (2.0)	$ (2.0)		$ (9.0)	$ (4.0)
Interest Rate Cash Flow Hedge Ineffectiveness is Immaterial	insignificant			insignificant			insignificant	insignificant
Commodity Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.0			$ 0.0			$ 2.0	$ (2.0)
Commodity Contract | Cash Flow Hedging [Member]
Derivative [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax							2.0	(4.0)
Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	3.0			2.0			7.0	6.0
Interest Rate Contract | Cash Flow Hedging [Member]
Derivative [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	1.0			0.0			3.0	(1.0)
Gas Purchased for Resale [Member] [Member] | Commodity Contract | Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net							(2.0)	2.0
Gas Purchased for Resale [Member] [Member] | Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred							0.5
Interest Expense [Member] | Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred							7.9
Interest Expense [Member] | Interest Rate Contract | Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ (3.0)			$ (2.0)			$ (7.0)	$ (6.0)
SCEG
Derivative [Line Items]
Interest Rate Cash Flow Hedge Ineffectiveness is Immaterial	insignificant			insignificant			insignificant	insignificant
Not Designated as Hedging Instrument [Member] | Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Deferred in Regulatory Accounts Effective Portion, Net	$ 1.0			$ (6.0)			$ 66.0	$ (30.0)
Not Designated as Hedging Instrument [Member] | Other Nonoperating Income (Expense) [Member] | Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income				(1.0)				(2.0)
Not Designated as Hedging Instrument [Member] | Interest Expense [Member] | Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income	(1.0)						(2.0)
Not Designated as Hedging Instrument [Member] | Interest Expense [Member] | Interest Rate Contract | Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Regulatory Accounts into Income, Estimated Net Amount to be Transferrred							(2.8)
Not Designated as Hedging Instrument [Member] | Other Income [Member] | Interest Rate Contract
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income							115.0
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				0.0			2.0	(1.0)
Designated as Hedging Instrument [Member] | Interest Expense [Member] | Interest Rate Contract | Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Regulatory Accounts into Income, Estimated Net Amount to be Transferrred							(1.0)
Derivative Instruments, Gain (Loss) Reclassified from Deferred Accounts into Income	$ 0.0			$ 0.0			$ (1.0)	$ (1.0)