Intangible Assets (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Intangible Assets
Intangible Assets, Gross	$ 226
Intangible Assets, Accumulated Amortization	146
Intangible Assets, Net	80
Amortization Expense
Actual, 2019	8
Estimated, 2020	7
Estimated, 2021	5
Estimated, 2022	5
Estimated, 2023	5
Estimated, 2024	5
Estimated, Thereafter	53
NUG contracts
Intangible Assets
Intangible Assets, Gross	124
Intangible Assets, Accumulated Amortization	46
Intangible Assets, Net	78
Amortization Expense
Actual, 2019	5
Estimated, 2020	5
Estimated, 2021	5
Estimated, 2022	5
Estimated, 2023	5
Estimated, 2024	5
Estimated, Thereafter	53
Coal contracts
Intangible Assets
Intangible Assets, Gross	102
Intangible Assets, Accumulated Amortization	100
Intangible Assets, Net	2
Amortization Expense
Actual, 2019	3
Estimated, 2020	2
Estimated, 2021	0
Estimated, 2022	0
Estimated, 2023	0
Estimated, 2024	0
Estimated, Thereafter	$ 0

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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