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Derivative Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Debt Instrument, Face Amount		$ 4,300,000,000	$ 4,550,000,000
Net payment on financial derivatives		(24,619,000)	0	$ 19,287,000
Energy commodity derivatives deposits		37,328,000	0
Energy commodity derivatives deposits		0	36,690,000
Interest Expense [Member]
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months		(2,200,000)
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 2019 [Member] \| Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative, Notional Amount		$ 250,000,000.0
Interest Rate Contract [Member] \| Future Debt Issuance 2019 [Member] \| Cash Flow Hedging [Member] \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability		8,400,000
Interest Rate Contract [Member] \| Future Debt Issuance 2019 [Member] \| Cash Flow Hedging [Member] \| Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		300,000
Interest Rate Contract [Member] \| Future Debt Issuance 2018 [Member] \| Cash Flow Hedging [Member]
Derivative [Line Items]
Derivative, Notional Amount		200,000,000.0
Net payment on financial derivatives		$ (24,600,000)
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