Retirement Plans, Components of Net Periodic Benefit Cost, Other Postretirement Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial gain	[1]	$ (7,322)	$ (25,776)	$ 15,013
Recognition of prior service credit (cost)	[1]	(2,317)	(3,760)	(4,763)
Other Postretirement Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost		0	0	0
Interest cost		24	27	29
Actuarial gain		(596)	(668)	(747)
Prior service cost		0	0	0
Net periodic benefit cost		(572)	(641)	(718)
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		144	52	(198)
Recognition of actuarial gain		596	668	747
Recognition of prior service credit (cost)		0	0	0
Adjustment for actual Medicare Part D reimbursement		0	0	(3)
Total recognized in other comprehensive income		740	720	546
Total recognized in net periodic benefit cost and other comprehensive income		$ 168	$ 79	$ (172)

[1] Actuarial gains (losses) are amortized into other income (expense). (See Note 11 - Retirement Plans)