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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, 2018
Dec. 31, 2017
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% 7.75% 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%