Financial Instruments and Risk Management (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 bbl / d $ / bbl	Dec. 31, 2017 USD ($) bbl / d	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of loss reclassified to interest expense		$ 2,963	$ 2,963	$ 3,000
Amount of loss expected to be reclassified to interest expense in the next 12 months		3,000
Loss deferred for fair value of interest rate derivative contracts, net of tax		8,400
Income taxes on deferred loss on fair value of interest rate derivative contracts		$ 4,500
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d		22,000
Fair value of derivative contract liability		$ 39,100	48,900
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts		$ (34,000)	(47,700)
Commodity Derivative Contracts [Member] | Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d	21,000
Average price | $ / bbl	54.88
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments			$ 14,200