COMMITMENTS AND CONTINGENCIES (COMMITMENTS) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) rental_payment_option	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 31, 2018 USD ($)
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Number of Different Rental Payments | rental_payment_option	5
Unrecorded Unconditional Purchase Obligation, Ownership Percentage	7.50%
2018	$ 204
2019	176
2020	153
2021	120
2022	98
Thereafter	1,051
Total	1,802
Rent expense	1	$ 2	$ 3
Fuel, Including Transportation
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	82
2019	83
2020	73
2021	43
2022	24
Thereafter	244
Total	549
Purchased Power
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	29
2019	0
2020	0
2021	0
2022	0
Thereafter	0
Total	29
Transmission
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	19
2019	19
2020	8
2021	4
2022	1
Thereafter	8
Total	59
Renewable Power Purchase Agreements
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	64
2019	64
2020	63
2021	63
2022	63
Thereafter	$ 668
Percentage of Purchase Power Obligations	100.00%
Total	$ 985
RES Performance-Based Incentives
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	8
2019	8
2020	7
2021	7
2022	7
Thereafter	46
Total	83
Operating Leases
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	1
2019	1
2020	1
2021	1
2022	1
Thereafter	3
Total	8
Land Easements and Rights-of-Way
Unrecorded Unconditional Purchase Obligation [Line Items]
2018	1
2019	1
2020	1
2021	2
2022	2
Thereafter	82
Total	89
Subsequent Event | Fuel, Including Transportation
Unrecorded Unconditional Purchase Obligation [Line Items]
2018				$ 4
2019				5
2020				5
2021				5
2022				5
Thereafter				$ 1
Maximum
Unrecorded Unconditional Purchase Obligation [Line Items]
Total	8
Minimum
Unrecorded Unconditional Purchase Obligation [Line Items]
Total	$ 3