Financial Instruments and Risk Management (Narrative) (Details) $ in Thousands	1 Months Ended	6 Months Ended			12 Months Ended
	May. 31, 2012 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($) item	Jun. 30, 2014 USD ($)	Dec. 31, 2014 item
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Repayments of debt			$ 450,000
Deferred loss charged to income			1,500	$ 1,500
Loss deferred for fair value of interest rate derivative contracts, tax			$ 7,100
Number of offsetting positions | item			0		0
10-Year Notes [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Repayments of debt	$ 350,000
Debt instrument term			10 years
Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Deferred loss charged to income		$ 1,500
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			$ 7,400	(36,900)
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			13,200
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			11	700
Foreign Exchange Derivative Contracts [Member] | Currency, U.S. Dollar
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Short-term derivative instruments			$ 8,000	$ 33,000