Segment Information - Reconciliation of Segment Information, Total Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
ASSETS
Segment Assets for Total Reportable Business Segments:	$ 8,528,950	$ 9,060,806	$ 8,777,361
Items Excluded from Segment Assets:
Recoverable Income Taxes	115,261	62,425
Total Consolidated Assets	8,528,950	$ 9,060,806	8,777,361
Intersegment Eliminations
ASSETS
Segment Assets for Total Reportable Business Segments:	(158,615)		(10,026)
Items Excluded from Segment Assets:
Total Consolidated Assets	(158,615)		(10,026)
Segment Reconciling Items
Items Excluded from Segment Assets:
Cash and Cash Equivalents	31,833		23,972
Recoverable Income Taxes	115,261		113,592
E&P | Operating Segments
ASSETS
Segment Assets for Total Reportable Business Segments:	6,785,652		6,638,207
Items Excluded from Segment Assets:
Total Consolidated Assets	6,785,652		6,638,207
Midstream | Operating Segments
ASSETS
Segment Assets for Total Reportable Business Segments:	1,754,819		2,011,616
Items Excluded from Segment Assets:
Total Consolidated Assets	$ 1,754,819		$ 2,011,616