Acquisitions and Divestitures (Details) - USD ($) $ in Thousands, shares in Millions	9 Months Ended	12 Months Ended
	Oct. 04, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Combinations [Abstract]
Proceeds from Sale of Producing Properties		$ 227,446	$ 226,768	$ 1,119,215
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
Stock Issued During Period, Shares, Acquisitions (in shares)	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
Business Combination, Separately Recognized Transactions [Line Items]
(Gain) Loss on Disposition of Assets		$ (174,562)	$ 99,096	$ (205,835)
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	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