Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Sales Revenue, Net [Member] \| Indiana [Member]
Percentage of revenue	69.00%	74.00%	67.00%	77.00%
Fixed-price Contract [Member]
Performance obligation	$ 662		$ 662
Percentage of revenue expected to be recognized by set date	54.00%		54.00%
Revenue, remaining performance obligation, expected timing of satisfaction, year	2020		2020
Revenue, remaining performance obligation, expected timing of satisfaction, explanation			We expect to recognize approximately 54% of this revenue through 2020, with the remainder recognized thereafter.
Index Priced Contracts [Member]
Performance obligation	$ 266		$ 266
Revenue, remaining performance obligation, expected timing of satisfaction, year		2021		2021
Revenue, remaining performance obligation, expected timing of satisfaction, explanation			We expect to recognize all of this revenue beginning in 2021.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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-Section 50
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Description of when remaining performance obligation is expected to be recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Year in which remaining performance obligation is expected to be recognized, in CCYY format.

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