Pension and Postretirement Benefit Plans - Components of net periodic benefit cost and other information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net periodic benefit cost (benefit)
Net periodic benefit cost (benefit)	$ (19)	$ (33)	$ (59)	$ (96)
Funded | Postretirement Benefits
Net periodic benefit cost (benefit)
Service cost	13	13	39	38
Interest cost	20	20	60	59
Expected return on plan assets	(21)	(21)	(63)	(63)
Amortization of prior service benefit	(10)	(13)	(30)	(39)
Amortization of net actuarial loss	15	14	45	42
Total non-operating expense (benefit)	4		12	(1)
Net periodic benefit cost (benefit)	17	13	51	37
Funded | United States | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	72	67	216	201
Interest cost	141	141	423	425
Expected return on plan assets	(272)	(259)	(816)	(777)
Amortization of prior service benefit	(6)	(5)	(18)	(17)
Amortization of net actuarial loss	126	97	378	291
Total non-operating expense (benefit)	(11)	(26)	(33)	(78)
Net periodic benefit cost (benefit)	61	41	183	123
Funded | International | Qualified and Non-qualified Pension Benefits
Net periodic benefit cost (benefit)
Service cost	37	33	110	100
Interest cost	40	38	120	112
Expected return on plan assets	(78)	(71)	(235)	(209)
Amortization of prior service benefit	(4)	(3)	(10)	(9)
Amortization of net actuarial loss	29	29	87	89
Total non-operating expense (benefit)	(13)	(7)	(38)	(17)
Net periodic benefit cost (benefit)	$ 24	$ 26	$ 72	$ 83