Derivative Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Debt Instrument, Face Amount		$ 4,550,000,000	$ 4,100,000,000
Net payment on financial derivatives		0	19,287,000	$ 42,908,000
Energy commodity derivatives deposits		36,690,000	49,899,000
Interest Expense [Member]
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months		(3,000,000)
4.25% Notes Due 2046 [Member]
Derivative [Line Items]
Debt Instrument, Face Amount		$ 500,000,000	500,000,000
Debt Instrument, Interest Rate, Stated Percentage		4.25%
Interest Rate Contract [Member] | Future Debt Issuance 2018 [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative, Notional Amount		$ 100,000,000.0
Derivative Asset, Fair Value, Gross Asset		12,200,000
Interest Rate Contract [Member] | 4.25% Notes Due 2046 [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250,000,000.0
Debt Instrument, Face Amount	$ 500,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Net payment on financial derivatives	$ 19,300,000
Commodity Contract [Member]
Derivative [Line Items]
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness		$ 2,400,000	$ 5,200,000	$ 1,000,000