Pension and Other Postretirement Benefit Plans Components of Net Periodic Pension Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Other Postretirement Benefit Plans [Abstract]
Defined Benefit Plan, Service Cost	$ 1,494	$ 1,456	$ 1,787
Defined Benefit Plan, Interest Cost	1,121	1,329	1,192
Defined Benefit Plan, Expected Return on Plan Assets	(950)	(1,117)	(986)
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	0	0	(96)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(77)	(81)	(85)
Defined Benefit Plan, Amortization of Gains (Losses)	440	593	727
Defined Benefit Plan, Net Periodic Benefit Cost	$ 2,028	$ 2,180	$ 2,539