Pension Benefits and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0
Interest cost	1,680	1,841
Expected return on plan assets	(2,169)	(2,309)
Amortization of prior service cost (credit)	0	0
Amortization of net loss (gain)	456	433
Net periodic (benefit) cost	(33)	(35)
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	39
Interest cost	15	18
Expected return on plan assets	0	0
Amortization of prior service cost (credit)	(400)	(373)
Amortization of net loss (gain)	0	(14)
Net periodic (benefit) cost	$ (365)	$ (330)