Other Postretirement Benefits (Changes in the Accumulated Postretirement Benefit Obligation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at	$ 1,377	$ 1,685
Interest cost	46	59	$ 66
Actuarial losses (gains)	7	5
Benefits paid	(360)	(372)
Projected benefit obligation at	1,070	1,377	1,685
Fair value of plan assets	0	0
Under funded status	(1,070)	(1,377)
Unamortized prior service cost	2	4
Unrecognized actuarial gain	(173)	(200)
Net amount recognized	(1,241)	(1,573)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at	7,202	8,149
Service cost	17	22	26
Interest cost	269	272	300
Actuarial losses (gains)	(150)	(923)
Benefits paid	(318)	(318)
Projected benefit obligation at	7,020	7,202	$ 8,149
Fair value of plan assets	0	0
Under funded status	(7,020)	(7,202)
Unamortized prior service cost	427	471
Unrecognized actuarial gain	2,382	2,679
Net amount recognized	$ (4,211)	$ (4,052)