Pension and Other Postretirement Benefits, Net Periodic Benefit Cost (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans [Member]
Components of net periodic benefit cost [Abstract]
Service cost		$ 112	$ 108	$ 123
Interest cost		142	135	149
Expected return on plan assets		(212)	(200)	(205)
Amortization of prior service cost (credits)		1	0	1
Amortization of net loss (gains)		61	52	68
Curtailment loss and special termination benefits	[1]	6	2	3
Net periodic benefit cost		110	97	139
Postretirement Benefit Plans [Member]
Components of net periodic benefit cost [Abstract]
Service cost		2	2	3
Interest cost		5	6	7
Expected return on plan assets		(4)	(4)	(5)
Amortization of prior service cost (credits)		(1)	(2)	(2)
Amortization of net loss (gains)		(2)	(2)	0
Curtailment loss and special termination benefits	[1]	0	0	0
Net periodic benefit cost		$ 0	$ 0	$ 3

[1] During the year ended December 31, 2017, the curtailment loss and special termination benefits charge is primarily due to a pension curtailment charge in connection with the decision to discontinue future service accruals for substantially all salaried employee defined benefit plans effective January 1, 2019.