Employee Benefits (Other Comprehensive Income, Other Postretirement Benefit Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net (gain) loss	$ (12,634)	$ 20,053	$ (5,429)
Prior service benefit	0	(32,697)	(824)
Amortization of:
Prior service benefit	9,657	7,407	6,574
Net gain	(6,776)	(4,965)	(8,622)
Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net (gain) loss	(10,586)	10,143	(8,884)
Prior service benefit	0	(32,697)	(824)
Amortization of:
Prior service benefit	6,151	3,901	3,068
Net gain	1,678	2,374	2,025
Total recognized in other comprehensive income	$ (2,757)	$ (16,279)	$ (4,615)