Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Sales Revenue, Net [Member] | Indiana [Member]
Percentage of revenue	72.00%	74.00%
Fixed-price Contract [Member]
Performance obligation	$ 756
Percentage of revenue expected to be recognized by set date	65.00%
Revenue, remaining performance obligation, expected timing of satisfaction, year	2020
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize approximately 65% 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.