Revenue Recognition (Narrative) (Details)	6 Months Ended
	Jun. 30, 2018
Upstream Revenues [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Number of days allowed for payment of invoiced amount	30 days
Marketing And Midstream Revenues [Member]
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Number of days allowed for payment of invoiced amount	30 days