Acquisitions and Divestitures (Details) - USD ($) $ in Thousands, shares in Millions	9 Months Ended	12 Months Ended
	Oct. 04, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combinations [Abstract]
Proceeds from Sale of Producing Properties		$ 226,768	$ 1,119,215	$ 192,807
Book value of assets held-for-sale		188,000
Book value of asset retirement obligations		41,000
Proceeds from Sales of Producing Properties		$ 73,000
Aggregated Purchase Price to Acquire Proved Crude Oil Properties and Related Assets				481,000
Stock Issued During Period, Shares, Acquisitions (in shares)	25	25
Payments to Acquire Businesses, Gross	$ 16,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	2,400,000
Cash Acquired from Acquisition	70,000
Purchase Price Allocation Net Decrease in Oil and Gas Properties		$ 35,000
Purchase Price Allocation Net Decrease in Deferred Income Taxes		32,000
Business Combination, Separately Recognized Transactions [Line Items]
Gain (Loss) on Disposition of Assets		(99,096)	$ 205,835	$ (8,798)
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed		5,000
Cash and Equivalents		70,411
Accounts Receivable, Net		77,073
Inventories		10,955
Other		10,640
Total Current Assets		169,079
Oil and Gas Properties (Successful Efforts Method)		3,815,207
Other Property, Plant and Equipment		21,824
Total Property, Plant and Equipment, Net		3,837,031
Other Assets		22,706
Total Assets		4,028,816
Accounts Payable		124,145
Accrued Taxes Payable		22,417
Other		743
Total Current Liabilities		147,305
Long-Term Debt	$ 164,000	163,829
Asset Retirement Obligations		163,144
Off-Market Transportation Contracts		39,720
Other Liabilities		28,645
Deferred Income Taxes		1,072,405
Total Liabilities		1,615,048
Total Consideration Transferred		$ 2,413,768