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Pension and Postretirement Plans (Narrative) (Details) - Defined Benefit Pension Plan [Member]
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
USD ($)
investment
country
Dec. 31, 2020
USD ($)
investment
Defined Benefit Plan Disclosure [Line Items]    
Percent of plan assets managed internally by the company 40.00%  
Percent of plan assets managed by investment companies 60.00%  
Number of investment companies actively managing plan assets | country 2  
Percentage of total plan assets 100.00% 100.00%
Berkshire Hathaway Common Stock [Member] | Investment Manager 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator 20.00%  
Alphabet and Berkshire Hathaway Common Stock [Member] | Investment Manager 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator 15.00%  
Foreign Investments [Member] | Maximum [Member] | Investment Manager 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, target allocation percentage of assets 30.00%  
Foreign Investments [Member] | Maximum [Member] | Investment Manager 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, target allocation percentage of assets 15.00%  
Fixed income securities [Member] | Minimum [Member] | Investment Manager 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, target allocation percentage of assets 10.00%  
Private investment fund [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of total plan assets 17.00% 18.00%
Single Equity Concentration [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined benefit plan, target allocation maximum percentage of assets, singular equity security, without prior approval by plan administrator 10.00% 10.00%
Value of investments | $ $ 885.3 $ 850.6
Percentage of total plan assets 30.00% 30.00%
Single Equity Concentration [Member] | Equity Securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Number of investments the company's pension plan held which individually exceed 10% of total plan assets 1 1
Single Equity Concentration [Member] | Private investment fund [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Number of investments the company's pension plan held which individually exceed 10% of total plan assets 1 1
Concentration In Single Entity, Type Of Industry, Foreign Country Or Individual Fund [Member] | Defined Benefit Plan Assets Total [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Minimum percentage of plan assets considered as significant concentrations in pension plans 10.00%