Financial Instruments and Risk Management (Narrative) (Details)	3 Months Ended		9 Months Ended
	Mar. 31, 2016 USD ($) item	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) bbl / d $ / bbl	Dec. 31, 2015 USD ($) item	May. 31, 2012 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Deferred loss charged to income	$ 741,000
Number of offsetting positions | item	0			0
10-Year Notes [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Note term	10 years
Face amount of notes					$ 350,000,000
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Daily production commitment (barrels per day) | bbl / d			5,000
Swap price | $ / bbl			45.30
Deferred loss charged to income			$ 2,100,000
Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts	$ 56,800,000
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	18,100,000
Loss deferred for fair value of interest rate derivative contracts	(6,300,000)
Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Increase (decrease) of income before taxes due to the impact of marking to market of derivative contracts	300,000	$ 38,000,000
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments	11,300,000	$ 15,500,000
Accounts Payable [Member] | Nondesignated [Member] | Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of foreign derivative contracts				$ 29,000
Accounts Receivable [Member] | Nondesignated [Member] | Commodity Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of commodity derivative contracts	65,518,000			$ 89,358,000
Accounts Receivable [Member] | Nondesignated [Member] | Foreign Exchange Derivative Contracts [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Fair value of commodity derivative contracts	$ 276,000