Retirement Plans, Summary of Funded Status at End of Year (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	$ 395,137,000	$ 358,197,000	$ 303,151,000
Benefit obligations	(508,694,000)	(442,861,000)	(381,483,000)
Funded status and amount recognized at end of year	(113,557,000)	(84,664,000)
SERP [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	0	0	0
Benefit obligations	(1,174,000)	(1,225,000)	(1,246,000)
Funded status and amount recognized at end of year	(1,174,000)	(1,225,000)
Other Postretirement Benefits [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	0	0	0
Benefit obligations	(629,000)	(662,000)	$ (743,000)
Funded status and amount recognized at end of year	$ (629,000)	$ (662,000)