Segment Information Reconciliation of Segment Information, Total Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
ASSETS
Segment assets for total reportable business segments	$ 8,122,229	$ 6,931,913	$ 9,019,597
Items excluded from segment assets:
Recoverable income taxes	20,178	31,523
Total Consolidated Assets	8,122,229	$ 6,931,913	9,019,597
Intersegment Eliminations [Member]
ASSETS
Segment assets for total reportable business segments	(14,770)		0
Items excluded from segment assets:
Total Consolidated Assets	(14,770)		0
Segment Reconciling Items [Member]
Items excluded from segment assets:
Cash and other investments	82,490		53,766
Recoverable income taxes	20,178		121,243
Discontinued Operations	0		2,493,816
E&P [Member] \| Operating Segments [Member]
ASSETS
Segment assets for total reportable business segments	6,034,660		6,350,772
Items excluded from segment assets:
Total Consolidated Assets	6,034,660		6,350,772
Midstream [Member] \| Operating Segments [Member]
ASSETS
Segment assets for total reportable business segments	1,999,671		0
Items excluded from segment assets:
Total Consolidated Assets	$ 1,999,671		$ 0

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