Property, Plant and Equipment and Other Intangibles (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Interest Costs Threshold For Capitalization Construction In Process	$ 500
Long-term Debt, Weighted Average Interest Rate, over Time	4.80%	4.90%	4.70%
Property, Plant and Equipment, Gross	$ 7,235,468	$ 6,783,737
Depreciation expense	$ 196,300	176,700	$ 164,100
Finite-Lived Intangible Asset, Useful Life	21 years
Amortization of Intangible Assets	$ 400	1,400	$ 2,700
Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	244,884	206,219
Capitalized Interest [Member] | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 57,300	$ 43,100
Minimum [Member]
Property, Plant and Equipment [Line Items]
Finite-Lived Intangible Asset, Useful Life	6 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Finite-Lived Intangible Asset, Useful Life	30 years