Financial Instruments and Risk Management (Narrative) (Details) bbl / d in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2018 USD ($) bbl / d contract item $ / bbl	Mar. 31, 2017 USD ($) bbl / d	Dec. 31, 2018 USD ($)	Dec. 31, 2017 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of foreign currency derivatives | contract	0
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			$ 2,200
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d	21	22
Average price | $ / bbl	54.88
Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of foreign derivative contracts		$ 200
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments		$ 18,500
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	$ 9,700
Income taxes on deferred loss on fair value of interest rate derivative contracts	$ 2,600
Notes Due 2022 [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Debt instrument term	10 years