Supplemental Compensation Plans, Assumptions Used to Determine Benefit Obligations (Details) - Pension Plan [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Salary Continuation Plan [Member]
Assumptions used to determine benefit obligations [Abstract]
Discount rate used to determine net periodic benefit cost for years ended	3.00%	4.10%	3.40%
Discount rate used to determine benefit obligations	2.30%	3.00%	4.10%
Future salary increases	6.20%	5.70%	5.70%
Directors' Retirement Plan [Member]
Assumptions used to determine benefit obligations [Abstract]
Discount rate used to determine net periodic benefit cost for years ended	2.40%	3.70%	3.00%
Discount rate used to determine benefit obligations	1.30%	2.40%	3.70%