Other Postretirement Benefits (Impact on Amounts Reported Due to a 1% Change in the Annual Medical Inflation Rate Issued) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefits [Abstract]
Effect on accumulated postretirement benefit obligation, 1% increase	$ 950	$ 975
Effect on accumulated postretirement benefit obligation, 1% decrease	(803)	(824)
Effect on annual service and interest costs, 1% increase	41	42
Effect on annual service and interest costs, 1% decrease	$ (34)	$ (35)