Reconciliation of Segment Information, Total Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
ASSETS
Segment assets for total reportable business segments	$ 8,209,036	$ 6,931,913	$ 8,979,568
Items excluded from segment assets:
Recoverable Income Taxes	40,024	31,523
Total Consolidated Assets	8,209,036	$ 6,931,913	8,979,568
Intersegment Eliminations [Member]
ASSETS
Segment assets for total reportable business segments	(12,926)		0
Items excluded from segment assets:
Total Consolidated Assets	(12,926)		0
Segment Reconciling Items [Member]
Items excluded from segment assets:
Cash and Cash Equivalents	42,672		281,148
Recoverable Income Taxes	40,024		101,501
Discontinued Operations	0		2,404,938
E&P [Member] | Operating Segments [Member]
ASSETS
Segment assets for total reportable business segments	6,256,132		6,191,981
Items excluded from segment assets:
Total Consolidated Assets	6,256,132		6,191,981
Midstream [Member] | Operating Segments [Member]
ASSETS
Segment assets for total reportable business segments	1,883,134		0
Items excluded from segment assets:
Total Consolidated Assets	$ 1,883,134		$ 0