Segment Information - Reconciliation of Segment Information, Total Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
ASSETS
Segment Assets for Total Reportable Business Segments:	$ 8,319,684	$ 9,060,806	$ 9,146,697
Items Excluded from Segment Assets:
Recoverable Income Taxes	114,440	62,425
Total Consolidated Assets	8,319,684	$ 9,060,806	9,146,697
Intersegment Eliminations
ASSETS
Segment Assets for Total Reportable Business Segments:	(153,991)		(3,041)
Items Excluded from Segment Assets:
Total Consolidated Assets	(153,991)		(3,041)
Segment Reconciling Items
Items Excluded from Segment Assets:
Cash and Cash Equivalents	19,607		32,611
Recoverable Income Taxes	114,440		113,592
E&P | Operating Segments
ASSETS
Segment Assets for Total Reportable Business Segments:	6,584,989		6,887,043
Items Excluded from Segment Assets:
Total Consolidated Assets	6,584,989		6,887,043
Midstream | Operating Segments
ASSETS
Segment Assets for Total Reportable Business Segments:	1,754,639		2,116,492
Items Excluded from Segment Assets:
Total Consolidated Assets	$ 1,754,639		$ 2,116,492