Significant Accounting Policies and Methods of Application - Changes in Amount of Goodwill (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Jun. 30, 2016
Goodwill [Roll Forward]
Goodwill - beginning balance	$ 1,827
Impairment	(14)	$ (14)
Goodwill - ending balance	1,813	1,813
Goodwill	1,827	1,813	$ 1,813
Goodwill, impairment loss, net of tax		9
Distribution Operations [Member]
Goodwill [Roll Forward]
Goodwill - beginning balance	1,640
Impairment	0
Goodwill - ending balance	1,640	1,640
Goodwill	1,640	1,640	1,640
Retail Operations [Member]
Goodwill [Roll Forward]
Goodwill - beginning balance	173
Impairment	0
Goodwill - ending balance	173	173
Goodwill	173	173	173
Midstream Operations [Member]
Goodwill [Roll Forward]
Goodwill - beginning balance	14
Impairment	(14)
Goodwill - ending balance	0	0
Goodwill	$ 14	$ 0	$ 0