Pension and Postretirement Benefit Plans - Components of net periodic benefit cost and other information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net periodic benefit cost (benefit)
Net periodic benefit cost (benefit)	$ (21)	$ (31)	$ (40)	$ (63)
Funded | Postretirement Benefits
Net periodic benefit cost (benefit)
Service cost	13	12	26	25
Interest cost	20	20	40	39
Expected return on plan assets	(21)	(21)	(42)	(42)
Amortization of prior service benefit	(10)	(13)	(20)	(26)
Amortization of net actuarial loss	15	14	30	28
Total non-operating expense (benefit)	4		8	(1)
Net periodic benefit cost (benefit)	17	12	34	24
Funded | United States | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	72	67	144	134
Interest cost	141	142	282	284
Expected return on plan assets	(272)	(259)	(544)	(518)
Amortization of prior service benefit	(6)	(6)	(12)	(12)
Amortization of net actuarial loss	126	97	252	194
Total non-operating expense (benefit)	(11)	(26)	(22)	(52)
Net periodic benefit cost (benefit)	61	41	122	82
Funded | International | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	37	34	73	67
Interest cost	40	37	80	74
Expected return on plan assets	(79)	(69)	(157)	(138)
Amortization of prior service benefit	(3)	(3)	(6)	(6)
Amortization of net actuarial loss	29	30	58	60
Total non-operating expense (benefit)	(13)	(5)	(25)	(10)
Net periodic benefit cost (benefit)	$ 24	$ 29	$ 48	$ 57