Financial Instruments and Risk Management (Narrative) (Details) bbl / d in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2017 USD ($) bbl / d item	Mar. 31, 2016 USD ($) bbl / d	Dec. 31, 2017 USD ($)	Dec. 31, 2016 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of loss reclassified to interest expense	$ 700	$ 700
Derivative loss	$ 741
Number of offsetting positions | item	0			0
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative loss			$ 2,100
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d	22	20
Fair value of derivative contract liability	$ 7,800
Fair value of derivative contract asset		$ 65,500
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts	(7,800)	56,800
Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of derivative contract asset	200	300
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments	18,500	$ 11,300
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	9,900
Income taxes on deferred loss on fair value of interest rate derivative contracts	$ 5,300