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Investments & NCI Sherwood Midstream Holdings (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2017	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	[1]	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Contribution of fixed assets to joint venture		$ 0		$ 328
Difference between carrying amount and underlying equity		1,000				$ 1,000
Equity method investments		$ 4,033				4,010
Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Fair Value Of Assets Contributed	$ 209		$ 10
Equity method investment, ownership percentage		81.00%
Ownership Interest In Assets Sold By Company In Affiliate		6.00%
Proceeds From Sale Of Ownership Interest In Assets Sold By Company In Affiliate		$ 15
Equity method investments		$ 151				$ 165
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	79.00%	62.00%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		18.90%
Sherwood Midstream | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	21.00%
Payments to acquire interest in joint venture	$ 44		$ 4
[1] Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5.

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