Nature of Operations and Basis of Consolidation Retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating, general and administrative	$ 74,345	$ 78,334	$ 294,803	$ 293,863	$ 290,718
Other Expense, net	$ (1,129)	(1,128)	(3,415)	(3,482)	826
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating, general and administrative		80,962	305,137	302,893	297,475
Other Expense, net		1,500	6,919	5,548	7,583
Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost (credit), effect of accounting change		$ (2,628)	$ (10,334)	$ (9,030)	$ (6,757)