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Retirement Plans - Rate and standard deviation assumptions (Details) - Non-contributory defined benefit pension plan
12 Months Ended
Dec. 31, 2019
Dec. 31, 2021
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Rate of Return Assumption (as a percent) 7.75%  
Cash equivalents    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Actual asset allocation (as a percent) 2.00%  
Cash equivalents | Maximum    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Allowable allocation percentages   35.00%
Cash equivalents | Minimum    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Allowable allocation percentages   0.00%
Equities | Maximum    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Allowable allocation percentages   65.00%
Equities | Minimum    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Allowable allocation percentages   55.00%
Fixed income    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Actual asset allocation (as a percent) 98.00%  
Fixed income | Maximum    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Allowable allocation percentages   40.00%
Fixed income | Minimum    
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan    
Allowable allocation percentages   20.00%