Retirement Plans, Components of Net Periodic Benefit Cost, Pension Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial loss	[1]	$ 31,755	$ 6,548	$ (7,322)
Pension Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost		7,671	7,364	7,622
Interest cost		15,630	16,493	15,499
Expected return on plan assets		(23,790)	(20,956)	(22,406)
Amortization of actuarial loss		2,399	1,438	2,890
Net periodic benefit cost		1,910	4,339	3,605
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		31,616	6,497	(7,396)
Recognition of actuarial loss		(2,399)	(1,438)	(2,890)
Total recognized in other comprehensive income		29,217	5,059	(10,286)
Total recognized in net periodic benefit cost and other comprehensive income		31,127	9,398	(6,681)
SERP [Member]
Components of net periodic benefit cost [Abstract]
Service cost		0	0	0
Interest cost		40	52	49
Expected return on plan assets		0	0	0
Amortization of actuarial loss		35	28	23
Net periodic benefit cost		75	80	72
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		55	73	(70)
Recognition of actuarial loss		(35)	(28)	(23)
Total recognized in other comprehensive income		20	45	(93)
Total recognized in net periodic benefit cost and other comprehensive income		$ 95	$ 125	$ (21)

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