Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of net periodic benefit cost [Abstract]
Contribution made to pension and postretirement plans	$ 0
Pension Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost	11	$ 11
Interest cost	326	329
Expected return on plan assets	(687)	(686)
Amortization of net loss (gain)	17	23
Amortization of prior service cost	0	0
Net periodic benefit	(333)	(323)
Other Postretirement Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost	26	28
Interest cost	54	56
Expected return on plan assets	(190)	(191)
Amortization of net loss (gain)	(89)	(86)
Amortization of prior service cost	23	23
Net periodic benefit	$ (176)	$ (170)