Derivative Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative [Line Items]
Good Faith and Margin Deposits with Broker-Dealers	$ 23,820,000		$ 0
Debt Instrument, Face Amount	4,319,000,000		4,300,000,000
Net receipt on financial derivatives	8,028,000	$ 0
Energy commodity derivatives deposits	0		37,328,000
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	(2,400,000)
Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount	100,000,000
Cash Flow Hedging [Member] | Interest Rate Contract [Member] | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative, Fair Value, Net	(4,500,000)
4.85% Notes Due 2049 [Member]
Derivative [Line Items]
Debt Instrument, Face Amount	$ 500,000,000.0		$ 0
Debt Instrument, Interest Rate, Stated Percentage	4.85%
4.85% Notes Due 2049 [Member] | Cash Flow Hedging [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 150,000,000
Net receipt on financial derivatives	$ (8,000,000)