Retirement Plans, Summary of Funded Status at End of Year (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	$ 358,197,000	$ 303,151,000	$ 294,995,000
Benefit obligations	(442,861,000)	(381,483,000)	(354,994,000)
Funded status and amount recognized at end of year	(84,664,000)	(78,332,000)
SERP [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	0	0	0
Benefit obligations	(1,225,000)	(1,246,000)	(1,412,000)
Funded status and amount recognized at end of year	(1,225,000)	(1,246,000)
Other Postretirement Benefits [Member]
Funded status at end of year [Abstract]
Fair value of plan assets	0	0	0
Benefit obligations	(662,000)	(743,000)	$ (679,000)
Funded status and amount recognized at end of year	$ (662,000)	$ (743,000)