Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
Employee Benefit Plans - Level 3 Fair Value Measurements (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in fair value of plan assets      
Fair value, January 1 $ 23,772    
Fair value, December 31 25,675 $ 23,772  
U.S. government and agency securities      
Change in fair value of plan assets      
Fair value, January 1 3,951    
Fair value, December 31 4,194 3,951  
Private real estate      
Change in fair value of plan assets      
Fair value, January 1 150    
Fair value, December 31 93 150  
Real estate commingled/mutual funds      
Change in fair value of plan assets      
Fair value, January 1 760    
Fair value, December 31 844 760  
Limited partnerships      
Change in fair value of plan assets      
Fair value, January 1 170    
Fair value, December 31 240 170  
Other investments      
Change in fair value of plan assets      
Fair value, January 1 830    
Fair value, December 31 824 830  
Level 3      
Change in fair value of plan assets      
Fair value, January 1 1,029 1,037 $ 962
Actual Return on Plan Assets Still Held at the Reporting Date 90 24 45
Purchases, Sales and Settlements (27) (32) 30
Fair value, December 31 1,092 1,029 1,037
Level 3 | U.S. government and agency securities      
Change in fair value of plan assets      
Fair value, January 1 10 11 11
Actual Return on Plan Assets Still Held at the Reporting Date 0 0 0
Purchases, Sales and Settlements (1) (1) 0
Fair value, December 31 9 10 11
Level 3 | Private real estate      
Change in fair value of plan assets      
Fair value, January 1 150 144 127
Actual Return on Plan Assets Still Held at the Reporting Date 8 1 14
Purchases, Sales and Settlements (65) 5 3
Fair value, December 31 93 150 144
Level 3 | Real estate commingled/mutual funds      
Change in fair value of plan assets      
Fair value, January 1 748 731 632
Actual Return on Plan Assets Still Held at the Reporting Date 63 21 37
Purchases, Sales and Settlements 20 (4) 62
Fair value, December 31 831 748 731
Level 3 | Limited partnerships      
Change in fair value of plan assets      
Fair value, January 1 38 49 65
Actual Return on Plan Assets Still Held at the Reporting Date 14 (2) (1)
Purchases, Sales and Settlements 33 (9) (15)
Fair value, December 31 85 38 49
Level 3 | Other investments      
Change in fair value of plan assets      
Fair value, January 1 83 102 127
Actual Return on Plan Assets Still Held at the Reporting Date 5 4 (5)
Purchases, Sales and Settlements (14) (23) (20)
Fair value, December 31 $ 74 $ 83 $ 102