Pension Benefits and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	1,692	1,841	3,372	3,682
Expected return on plan assets	(2,164)	(2,309)	(4,333)	(4,618)
Amortization of prior service credit	0	0	0	0
Amortization of net loss (gain)	457	433	913	866
Net periodic (benefit) cost	(15)	(35)	(48)	(70)
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	17	17	38	56
Interest cost	16	13	31	31
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	(400)	(407)	(801)	(780)
Amortization of net loss (gain)	0	(17)	0	(31)
Net periodic (benefit) cost	$ (367)	$ (394)	$ (732)	$ (724)