Financial Instruments and Risk Management (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 bbl / d	Dec. 31, 2016 USD ($) bbl / d	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	May 31, 2012 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of loss reclassified to interest expense		$ 2,963,000	$ 2,963,000	$ 2,963,000
Amount of loss expected to be reclassified to interest expense in the next 12 months		2,963,000
Loss deferred for fair value of interest rate derivative contracts, net of tax		15,926,000
Income tax on deferred loss on fair value of interest rate derivative contracts		$ 5,574,000
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d		20,000
Fair value of derivative contract liability		$ 48,900,000
Fair value of derivative contract asset			89,400,000
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts		(47,703,000)	77,300,000
Commodity Derivative Contracts [Member] | Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d	22,000
Interest Rate Swap [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Face amount of notes					$ 350,000,000
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments		14,200,000	4,800,000
Accounts Payable [Member] | Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of foreign derivative contracts		73,000	29,000
Accounts Payable [Member] | Nondesignated [Member] | Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of foreign derivative contracts		48,864,000
Accounts Payable [Member] | Nondesignated [Member] | Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of foreign derivative contracts		$ 73,000	29,000
Accounts Receivable [Member] | Nondesignated [Member] | Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of derivative contract asset			$ 89,358,000