Derivative Financial Instruments (Narrative) (Details) $ in Thousands, bbl in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) bbl	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) bbl	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Energy commodity derivatives deposits, current asset	$ 4,492		$ 4,492		$ 49,899
Energy commodity derivatives deposits	119		119		$ 0
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			(3,000)
Commodity Contract [Member]
Derivative [Line Items]
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	$ 300	$ 1,900	$ 1,700	$ 4,200
Fair Value Hedging [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	0.7		0.7
Fair Value Hedging [Member] | Commodity Contract [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	0.7		0.7
Future Debt Issuance 2018 [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 100,000		$ 100,000
Derivative Asset, Fair Value, Gross Asset	$ 12,600		$ 12,600