Financial Instruments and Risk Management (Narrative) (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	May. 31, 2012 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Dec. 31, 2016 bbl / d $ / bbl	Dec. 31, 2014 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Repayments of debt				$ 450,000
Deferred loss charged to income				2,200	$ 2,200
Loss deferred for fair value of interest rate derivative contracts, tax				6,900
Impairment of assets			$ 2,300,974	$ 2,300,974
Number of offsetting positions | item				0			0
10-Year Notes [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Repayments of debt	$ 350,000
Debt instrument term				10 years
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d						20,000
Swap price | $ / bbl						52.01
Interest expense due to deferred loss on interest rate swaps expected during the remaining nine months of 2015		$ 700
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				$ 24,200	(17,200)
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			12,800	12,800
Foreign Exchange 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				22	200
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments			$ 6,200	$ 6,200	$ 15,000