Pension and Postretirement Plans (Total Benefit/Cost) (Details 1) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net Periodic Cost (Benefit)	$ 4,940	$ 4,591	$ 4,591	$ 4,591	$ 4,914	$ 18,687	$ 20,461
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4,940				4,914
Interest cost	11,255				11,986
Expected return on assets	(32,486)				(30,337)
Amortization of prior service cost (credit)	42				43
Recognized actuarial loss (gain)	(1,046)				(1,294)
Net Periodic Benefit	(17,295)				(14,688)
Curtailment gain	(806)				0
Net Periodic Cost (Benefit)	(18,101)				(14,688)
Supplemental Executive Retirement Plan (SERP) [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	205				214
Interest cost	966				1,058
Amortization of prior service cost (credit)	78				114
Recognized actuarial loss (gain)	601				444
Net Periodic Cost (Benefit)	1,850				1,830
Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	268				257
Interest cost	170				195
Amortization of prior service cost (credit)	(44)				(37)
Recognized actuarial loss (gain)	(922)				(973)
Net Periodic Cost (Benefit)	$ (528)				$ (558)