Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									$ 2,525	$ 2,086	$ 1,413
Operating Revenues	$ 1,175	$ 951	$ 816	$ 920	$ 809	$ 737	$ 811	$ 846	3,862	3,203	2,436
Exploration and Production [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									2,525	2,086	1,413
Operating Revenues									2,551	2,105	1,435
Midstream Services [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									3,745	3,198	2,569
Operating Revenues									1,311	1,098	1,001
Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									(2,408)	(2,081)	(1,546)
Intersegment Eliminations [Member] | Exploration and Production [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									26	19	22
Intersegment Eliminations [Member] | Midstream Services [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									(2,434)	(2,100)	(1,568)
Gas Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									1,998	1,793	1,273
Gas Sales [Member] | Exploration and Production [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									1,974	1,775	1,252
Gas Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									24	18	21
Oil Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									196	102	69
Oil Sales [Member] | Exploration and Production [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									193	101	69
Oil Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									3	1
NGL Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									352	206	92
NGL Sales [Member] | Exploration and Production [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									353	206	92
NGL Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									(1)
Marketing [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									1,222	972	864
Marketing [Member] | Midstream Services [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									3,497	2,867	2,191
Marketing [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									(2,275)	(1,895)	(1,327)
Gas Gathering [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									89	126	138
Gas Gathering [Member] | Midstream Services [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									248	331	378
Gas Gathering [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									(159)	(205)	$ (240)
Other [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues									5	4
Other [Member] | Exploration and Production [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue									$ 5	$ 4