Retirement Plans, Components of Net Periodic Benefit Cost, Other Postretirement Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial gain	[1]	$ 31,755	$ 6,548	$ (7,322)
Other Postretirement Benefits [Member]
Components of net periodic benefit cost [Abstract]
Interest cost		22	31	24
Amortization of actuarial gain		(522)	(540)	(596)
Net periodic benefit cost		(500)	(509)	(572)
Other changes in benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		84	(22)	144
Recognition of actuarial gain		522	540	596
Total recognized in other comprehensive income		606	518	740
Total recognized in net periodic benefit cost and other comprehensive income		$ 106	$ 9	$ 168

[1] Actuarial gains (losses) are amortized into other income (expense). (See Note 12 – Retirement Plans)