Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Sales Revenue, Net [Member] | Indiana [Member]
Percentage of revenue	78.00%	72.00%
Fixed-price Contract [Member]
Performance obligation	$ 508
Percentage of revenue expected to be recognized by set date	74.00%
Revenue, remaining performance obligation, expected timing of satisfaction, year	2021
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize approximately 74% of this revenue through 2021, with the remainder recognized thereafter.
Index Priced Contracts [Member]
Performance obligation	$ 266
Revenue, remaining performance obligation, expected timing of satisfaction, year		2021
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize all of this revenue beginning in 2021.