Segment Information - Reconciliation of Segment Information, Total Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
ASSETS
Segment Assets for Total Reportable Business Segments:	$ 9,286,258	$ 8,592,170	$ 8,209,036
Items Excluded from Segment Assets:
Recoverable Income Taxes	11,184	149,481
Total Consolidated Assets	9,286,258	$ 8,592,170	8,209,036
Intersegment Eliminations
ASSETS
Segment Assets for Total Reportable Business Segments:	3,735		(12,926)
Items Excluded from Segment Assets:
Total Consolidated Assets	3,735		(12,926)
Segment Reconciling Items
Items Excluded from Segment Assets:
Cash and Cash Equivalents	5,484		42,672
Recoverable Income Taxes	11,184		40,024
E&P | Operating Segments
ASSETS
Segment Assets for Total Reportable Business Segments:	7,087,782		6,256,132
Items Excluded from Segment Assets:
Total Consolidated Assets	7,087,782		6,256,132
Midstream | Operating Segments
ASSETS
Segment Assets for Total Reportable Business Segments:	2,178,073		1,883,134
Items Excluded from Segment Assets:
Total Consolidated Assets	$ 2,178,073		$ 1,883,134