Financial Instruments and Risk Management (Narrative) (Details) bbl / d in Thousands, $ in Thousands	6 Months Ended
	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) bbl / d contract item $ / contract	Jun. 30, 2016 USD ($) bbl / d	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		$ 1,500	$ 1,500
Derivative loss		$ 1,482
Number of offsetting positions | item		0		0
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative loss	$ 1,500
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d		22	25
Average price | $ / contract		50.41
Fair value of derivative contract asset		$ 18,300	$ 1,700
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts		(14,900)	2,600
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			$ 5,800
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		9,400
Income taxes on deferred loss on fair value of interest rate derivative contracts		$ (5,100)