Annual report pursuant to Section 13 and 15(d)

Employee Benefit Plans - Level 3 Fair Value Measurements (Details)

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Employee Benefit Plans - Level 3 Fair Value Measurements (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Change in fair value of plan assets      
Fair value, January 1 $ 23,475    
Fair value, December 31 25,836 $ 23,475  
U.S. government and agency securities      
Change in fair value of plan assets      
Fair value, January 1 4,451    
Fair value, December 31 5,301 4,451  
Private real estate      
Change in fair value of plan assets      
Fair value, January 1 5    
Fair value, December 31 0 5  
Real estate commingled/mutual funds      
Change in fair value of plan assets      
Fair value, January 1 898    
Fair value, December 31 945 898  
Limited partnerships      
Change in fair value of plan assets      
Fair value, January 1 240    
Fair value, December 31 263 240  
Other investments      
Change in fair value of plan assets      
Fair value, January 1 1,045    
Fair value, December 31 1,037 1,045  
Level 3      
Change in fair value of plan assets      
Fair value, January 1 1,569 1,092 $ 1,029
Actual Return on Plan Assets Still Held at the Reporting Date 39 33 90
Purchases, Sales and Settlements 53 444 (27)
Fair value, December 31 1,661 1,569 1,092
Level 3 | U.S. government and agency securities      
Change in fair value of plan assets      
Fair value, January 1 9 9 10
Actual Return on Plan Assets Still Held at the Reporting Date 0 0 0
Purchases, Sales and Settlements (1) 0 (1)
Fair value, December 31 8 9 9
Level 3 | Private real estate      
Change in fair value of plan assets      
Fair value, January 1 5 93 150
Actual Return on Plan Assets Still Held at the Reporting Date 0 (7) 8
Purchases, Sales and Settlements (5) (81) (65)
Fair value, December 31 0 5 93
Level 3 | Real estate commingled/mutual funds      
Change in fair value of plan assets      
Fair value, January 1 885 831 748
Actual Return on Plan Assets Still Held at the Reporting Date 33 52 63
Purchases, Sales and Settlements 9 2 20
Fair value, December 31 927 885 831
Level 3 | Limited partnerships      
Change in fair value of plan assets      
Fair value, January 1 82 85 38
Actual Return on Plan Assets Still Held at the Reporting Date 0 (12) 14
Purchases, Sales and Settlements 8 9 33
Fair value, December 31 90 82 85
Level 3 | Other investments      
Change in fair value of plan assets      
Fair value, January 1 588 74 83
Actual Return on Plan Assets Still Held at the Reporting Date 6 0 5
Purchases, Sales and Settlements 42 514 (14)
Fair value, December 31 $ 636 $ 588 $ 74