Derivative Financial Instruments (Narrative) (Details) bbl in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) bbl	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) bbl	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) bbl
Derivative [Line Items]
Energy commodity derivatives deposits, current asset	$ 30,559,000		$ 30,559,000		$ 0
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			(3,000,000)
Debt Instrument, Face Amount	$ 4,300,000,000.0		4,300,000,000.0		$ 3,400,000,000.0
Payments for (Proceeds from) Hedge, Financing Activities			$ 19,287,000	$ 42,908,000
Fair Value Hedging [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	0.7		0.7
Fair Value Hedging [Member] | NYMEX Commodity Contracts [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | bbl	0.7		0.7		0.7
Cumulative Gains from Fair Value Hedges	$ 17,600,000		$ 17,600,000		$ 27,900,000
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	300,000	$ (1,700,000)	4,500,000	$ 4,600,000
Future Debt Issuance 2018 [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount	100,000,000		100,000,000
Derivative Liability, Fair Value, Gross Liability	3,500,000		3,500,000
Five Point Zero Percentage Notes Due Two Thousand Twenty Six [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount	250,000,000		250,000,000
Debt Instrument, Face Amount	$ 500,000,000		$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.25%		4.25%
Payments for (Proceeds from) Hedge, Financing Activities	$ 19,300,000
Maximum Length of Time Hedged in Interest Rate Cash Flow Hedge	10 years