Financial Instruments and Risk Management (Narrative) (Details) bbl / d in Thousands, $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) bbl / d contract item $ / bbl	Sep. 30, 2016 USD ($) bbl / d	Dec. 31, 2018 bbl / d $ / bbl	Dec. 31, 2016 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of open contracts | contract		0
Impairment of assets			$ 95,088
Amount of loss reclassified to interest expense		$ 2,200	$ 2,200
Derivative loss		$ 2,222
Number of offsetting positions | item		0			0
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative loss	$ 700
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d		22	25
Average price | $ / bbl		50.41
Fair value of derivative contract liability		$ 3,200
Fair value of derivative contract asset			$ 200
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts		(3,200)	3,900
Commodity Derivative Contracts [Member] | Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d				6
Average price | $ / bbl				51.83
Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of foreign derivative contracts			100
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments			$ 25,200
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		8,900
Income taxes on deferred loss on fair value of interest rate derivative contracts		$ 4,800