Retirement Plans, Schedule of Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	$ 442,861	$ 381,483
Service cost	7,671	7,364	$ 7,622
Interest cost	15,630	16,493	15,499
Actuarial loss (gain)	58,851	49,478
Gross benefits paid	(11,029)	(11,957)
Settlements	(5,290)	0
Benefit obligation at end of year	508,694	442,861	381,483
Accumulated benefit obligation at end of year	479,999	417,981
SERP [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	1,225	1,246
Service cost	0	0	0
Interest cost	40	52	49
Actuarial loss (gain)	55	72
Gross benefits paid	(146)	(145)
Settlements	0	0
Benefit obligation at end of year	1,174	1,225	1,246
Accumulated benefit obligation at end of year	1,174	1,225
Other Postretirement Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	662	743
Service cost	0	0
Interest cost	22	31	24
Actuarial loss (gain)	84	(22)
Gross benefits paid	(139)	(90)
Settlements	0	0
Benefit obligation at end of year	629	662	$ 743
Accumulated benefit obligation at end of year	$ 629	$ 662