Retirement Plans, Components of Net Periodic Benefit Cost, Other Postretirement Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial gain	[1]	$ (25,776)	$ 15,013	$ (19,482)
Recognition of prior service credit (cost)	[1]	(3,760)	(4,763)	(6,963)
Other Postretirement Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost		0	0	0
Interest cost		27	29	36
Actuarial loss (gain)		(668)	(747)	(793)
Prior service cost		0	0	0
Net periodic benefit cost		(641)	(718)	(757)
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain		52	(198)	(322)
Recognition of actuarial gain		668	747	793
Recognition of prior service credit (cost)		0	0	0
Adjustment for actual Medicare Part D reimbursement		0	(3)	(3)
Total recognized in other comprehensive income		720	546	468
Total recognized in net periodic benefit cost and other comprehensive income		$ 79	$ (172)	$ (289)

[1] Actuarial gains (losses) are amortized into costs of sales and operating expenses or selling, general and administrative expenses as appropriate. (See Note 10 - Retirement Plans).