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Commitments and Contingencies (Details)
 9 Months Ended
Sep. 30, 2021
USD ($)
location
Dec. 31, 2020
USD ($)
location
Commitments and Contingencies Disclosure [Abstract]    
Environmental liability $ 52,000,000 $ 54,000,000
Current environmental liability $ 15,000,000 $ 15,000,000
Known cleanup and remediation locations and projects | location 97 100
Number of sites - representative sample | location 19  
Liability associated with those sites $ 40,000,000  
Environmental locations representative sample liability payout period, in years 5 years  
Responsible locations with another party | location 8  
Self-insured injury/damage to third parties - and above, per occurrence $ 75,000,000  
Self-insured injury/damage to third party - up to 800,000,000  
Self-insured injury/damage to third parties - and above, per occurrence for specific perils $ 1,100,000,000  
Percentage of potential losses covered 87.00%  
Self-insured NS owned property - and above, per occurrence $ 75,000,000  
Self-insured NS owned property - up to $ 275,000,000