New Accounting Principles and Recent Accounting Pronouncements (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain tax effects from accumulated other comprehensive loss		$ (30,237)
Accounting Standards Update 2016-02 [Member] | Liabilities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement, expected effect		(160,000)
Accounting Standards Update 2016-02 [Member] | Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement, expected effect		800,000
Retirement and Postretirement Benefit Plan Adjustments [Member] | Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain tax effects from accumulated other comprehensive loss	$ 28,400
Deferred Loss On Interest Rate Derivative Hedges [Member] | Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain tax effects from accumulated other comprehensive loss	$ 1,800
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain tax effects from accumulated other comprehensive loss		30,237
Retained Earnings [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement, expected effect		$ 160,000