Retirement Plans, Summary of Change in Plan Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 358,197,000	$ 303,151,000
Actual return on plan assets	51,024,000	63,939,000
Employer contribution	2,235,000	3,064,000
Gross benefits paid	(11,029,000)	(11,957,000)
Settlements	(5,290,000)	0
Fair value of plan assets at end of year	395,137,000	358,197,000
SERP [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	146,000	145,000
Gross benefits paid	(146,000)	(145,000)
Settlements	0	0
Fair value of plan assets at end of year	0	0
Other Postretirement Benefits [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	139,000	90,000
Gross benefits paid	(139,000)	(90,000)
Settlements	0	0
Fair value of plan assets at end of year	$ 0	$ 0