Financial Instruments and Risk Management (Nonrecurring Fair Value Measurements) (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Pretax (Noncash) Impairment Loss	$ 2,300,974	$ 2,300,974
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets: Impaired proved properties	1,449,028	1,449,028
Carrying Value [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Book Value Prior to Impairment	3,750,002	3,750,002
Gulf of Mexico [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Pretax (Noncash) Impairment Loss	144,800	144,800
Gulf of Mexico [Member] | Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets: Impaired proved properties	216,602	216,602
Gulf of Mexico [Member] | Carrying Value [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Book Value Prior to Impairment	361,402	361,402
Western Canada [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Pretax (Noncash) Impairment Loss	683,574	683,574
Western Canada [Member] | Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets: Impaired proved properties	23,526	23,526
Western Canada [Member] | Carrying Value [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Book Value Prior to Impairment	707,100	707,100
Malaysia [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Pretax (Noncash) Impairment Loss	1,472,600	1,472,600
Malaysia [Member] | Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets: Impaired proved properties	1,208,900	1,208,900
Malaysia [Member] | Carrying Value [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Book Value Prior to Impairment	$ 2,681,500	$ 2,681,500