Retirement Plans, Components of Net Periodic Benefit Cost, Other Postretirement Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial gain	[1]	$ 6,548	$ (7,322)	$ (25,776)
Recognition of prior service credit (cost)	[1]	(926)	(2,317)	(3,760)
Other Postretirement Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost		0	0	0
Interest cost		31	24	27
Actuarial gain		(540)	(596)	(668)
Prior service cost		0	0	0
Net periodic benefit cost		(509)	(572)	(641)
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		(22)	144	52
Recognition of actuarial gain		540	596	668
Recognition of prior service credit (cost)		0	0	0
Total recognized in other comprehensive income		518	740	720
Total recognized in net periodic benefit cost and other comprehensive income		$ 9	$ 168	$ 79

[1] The discount rate for the Higman pension plan in 2018 was changed from 4.13% as of February 14, 2018 and 4.02% as of March 31, 2018 to 4.40% as of December 31, 2018.Actuarial gains (losses) are amortized into other income (expense). (See Note 11 – Retirement Plans)