Financial Instruments and Risk Management (Narrative) (Details) CAD in Billions	3 Months Ended			9 Months Ended
	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Jun. 30, 2016 CAD	Dec. 31, 2015 USD ($) item	May 31, 2012 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Deferred loss charged to income				$ 1,500,000	$ 1,500,000
Number of offsetting positions | item				0			0
Impairment Expense			$ 2,300,974,000	$ 95,088,000	2,300,974,000
10-Year Notes [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Note term				10 years
Face amount of notes								$ 350,000,000
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Deferred loss charged to income	$ 700,000
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts				$ (3,900,000)	7,400,000
Interest Rate Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Loss deferred for fair value of interest rate derivative contracts, net of tax				10,800,000
Income tax on deferred loss on fair value of interest rate derivative contracts				5,800,000
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments			$ 6,200,000	25,200,000	$ 6,200,000
Forward Sale Contracts [Member] | Currency, Canadian Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain charged to income		$ 26,800,000
Short-term derivative instruments | CAD						CAD 1.0
Accounts Receivable [Member] | Nondesignated [Member] | Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of commodity derivative contracts				192,000			$ 89,358,000
Accounts Receivable [Member] | Nondesignated [Member] | Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of commodity derivative contracts				$ 142,000
Syncrude Canada Ltd. [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of interest in oil and gas property sold during period		5.00%