Fair Value Measurements (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Proved Oil and Gas Properties, Other Property, Plant and Equipment and Other Assets [Abstract]
Proved Oil and Gas Properties and Other Assets Carrying Amount		$ 1,459,000
Proved Oil and Gas Properties and Other Assets Written Down During the Period - Fair value at End of Period		274,000
Pretax Impairment Charges for Proved Oil and Gas Properties and Other Property, Plant and Equipment		1,185,000
Pretax Impairment Changes For Other Assets		72,000
Debt Disclosure [Abstract]
Aggregate Principal Amount of Current and Long-Term Debt		5,640,000	$ 5,140,000
Estimated Fair Value of Debt		6,314,000	5,452,000
Commodity Contract [Member] | Crude Oil | Price Swaps
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	20,316
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	16,190	23,266
Commodity Contract [Member] | Crude Oil | Price Swaps | Fair Value, Inputs, Level 1 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0	0
Commodity Contract [Member] | Crude Oil | Price Swaps | Fair Value, Inputs, Level 2 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	20,316
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	16,190	23,266
Commodity Contract [Member] | Crude Oil | Price Swaps | Fair Value, Inputs, Level 3 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0	0
Commodity Contract [Member] | Crude Oil | Roll Differential Swap
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	816
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	5,074
Commodity Contract [Member] | Crude Oil | Roll Differential Swap | Fair Value, Inputs, Level 1 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Crude Oil | Roll Differential Swap | Fair Value, Inputs, Level 2 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	816
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	5,074
Commodity Contract [Member] | Crude Oil | Roll Differential Swap | Fair Value, Inputs, Level 3 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Natural Gas Liquids | Price Swaps
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	4,394	3,401
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	943
Commodity Contract [Member] | Natural Gas Liquids | Price Swaps | Fair Value, Inputs, Level 1 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Natural Gas Liquids | Price Swaps | Fair Value, Inputs, Level 2 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	4,394	3,401
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	943
Commodity Contract [Member] | Natural Gas Liquids | Price Swaps | Fair Value, Inputs, Level 3 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Natural Gas | Price Swaps
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	509
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	4,405
Commodity Contract [Member] | Natural Gas | Price Swaps | Fair Value, Inputs, Level 1 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Natural Gas | Price Swaps | Fair Value, Inputs, Level 2 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	509
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	4,405
Commodity Contract [Member] | Natural Gas | Price Swaps | Fair Value, Inputs, Level 3 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]	0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Natural Gas | Basis Swaps
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]		970
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	5,389
Commodity Contract [Member] | Natural Gas | Basis Swaps | Fair Value, Inputs, Level 1 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]		0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Commodity Contract [Member] | Natural Gas | Basis Swaps | Fair Value, Inputs, Level 2 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]		970
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	5,389
Commodity Contract [Member] | Natural Gas | Basis Swaps | Fair Value, Inputs, Level 3 [Member]
Financial Assets: [Abstract]
Assets, Fair Value Disclosure	[1]		0
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure	[2]	0
Assets From Price Risk Management Activities
Financial Assets: [Abstract]
Assets, Fair Value Disclosure		18,000	1,000
Liabilities From Price Risk Management Activities
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure		23,000	$ 20,000
Other Liabilities
Financial Liabilities:
Financial and Nonfinancial Liabilities, Fair Value Disclosure		$ 1,000

[1] $18 million and $1 million are included in "Current Assets - Assets from Price Risk Management Activities" at September 30, 2020 and December 31, 2019, respectively, on the Condensed Consolidated Balance Sheets.

[2] $23 million and $20 million is included in "Current Liabilities - Liabilities from Price Risk Management Activities" at September 30, 2020 and December 31, 2019, respectively, on the Condensed Consolidated Balance Sheets. $1 million is included in "Other Liabilities" at September 30, 2020, on the Condensed Consolidated Balance Sheets.