Retirement Plans (Income (Loss) Estimated to be Amortized from Accumulated Other Comprehensive Income (Loss) into Net Periodic Cost) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Twelve Months
Prior service (costs) credits	$ 0
Net gain (loss)	(448)
Total	(448)
Pension Plan
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Twelve Months
Prior service (costs) credits	0
Net gain (loss)	(561)
Total	(561)
Postretirement Plan
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Twelve Months
Prior service (costs) credits	0
Net gain (loss)	113
Total	113
SERP
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Twelve Months
Prior service (costs) credits	0
Net gain (loss)	0
Total	$ 0