Revenue (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Sales Revenue, Net [Member] | Indiana [Member]
Percentage of revenue	73.00%	68.00%
Fixed-price Contract [Member]
Performance obligation	$ 840
Percentage of revenue expected to be recognized by set date	68.00%
Revenue, remaining performance obligation, expected timing of satisfaction, year	2020
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize approximately 68% of this revenue through 2020, with the remainder recognized thereafter.
Index Priced Contracts [Member]
Performance obligation	$ 330
Revenue, remaining performance obligation, expected timing of satisfaction, year	2020
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize all of this income beginning in 2020.