Pension Benefit and Retirement Health and Life Insurance Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0
Interest cost	7,356	7,530	$ 7,523
Expected return of plan assets	(9,221)	(10,808)	(11,148)
Amortization of prior service cost (credit)	0	0
Amortization of net loss	1,755	1,784	1,690
Settlement charge	0	0	57
Net periodic benefit cost (benefit)	(110)	(1,494)	(1,878)
Retirement Health and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	133	411
Interest cost	71	75	216
Expected return of plan assets	0	0
Amortization of prior service cost (credit)	(1,602)	(1,489)	(248)
Amortization of net loss	0	(47)	(12)
Settlement charge	0	0
Net periodic benefit cost (benefit)	$ (1,451)	$ (1,328)	$ 367