Fair Value Measurements (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Financial Assets:
Financial Assets: Natural Gas Options/Collars	[1]	$ 1,000
Financial Liabilities:
Financial Liabilities: Crude Oil Swaps	[2]	36,000
Financial Liabilities: Natural Gas Swaps	[2]	4,000
Financial Liabilities: Crude Oil Options/Swaptions	[2]	22,000
Proved oil and gas properties, other property, plant and equipment and other assets, carrying amount		778,000	$ 9,154,000
Proved oil and gas properties, other property, plant and equipment and other assets written down during the period - fair value at end of period		587,000	2,828,000
Pretax impairment charges for proved oil and gas properties, other property, plant and equipment and other assets		191,000	6,326,000
Pretax impairment charges for obsolete inventory		61,000
Pretax impairment charges for firm commitment contracts		138,000
Net of Tax Impairment Charges for Proved Oil and Gas Properties, Other Property, Plant and Equipment and Other Assets			4,141,000
Aggregate Principal Amount of Current and Long-Term Debt		6,990,000	6,390,000
Fair Value of Debt		7,190,000	$ 6,524,000
Fair Value, Inputs, Level 1 [Member]
Financial Assets:
Financial Assets: Natural Gas Options/Collars	[1]	0
Financial Liabilities:
Financial Liabilities: Crude Oil Swaps	[2]	0
Financial Liabilities: Natural Gas Swaps	[2]	0
Financial Liabilities: Crude Oil Options/Swaptions	[2]	0
Fair Value, Inputs, Level 2 [Member]
Financial Assets:
Financial Assets: Natural Gas Options/Collars	[1]	1,000
Financial Liabilities:
Financial Liabilities: Crude Oil Swaps	[2]	36,000
Financial Liabilities: Natural Gas Swaps	[2]	4,000
Financial Liabilities: Crude Oil Options/Swaptions	[2]	22,000
Fair Value, Inputs, Level 3 [Member]
Financial Assets:
Financial Assets: Natural Gas Options/Collars	[1]	0
Financial Liabilities:
Financial Liabilities: Crude Oil Swaps	[2]	0
Financial Liabilities: Natural Gas Swaps	[2]	0
Financial Liabilities: Crude Oil Options/Swaptions	[2]	$ 0

[1] At December 31, 2016, $1 million is included in "Other Assets" on the Consolidated Balance Sheets.

[2] At December 31, 2016, $62 million is included in "Current Liabilities - Liabilities from Price Risk Management Activities" on the Consolidated Balance Sheets.