Employee Pension and Other Postretirement Benefits - Periodic Benefit Costs, Forecasted (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 17,386	$ 16,593	$ 15,812	$ 14,885
Amortization period of net actuarial losses		9 years
Excess of market value of plan assets		$ 107
Assetsmoothingperiodofnonamortizing amount		4 years
Amortization period of prior service credit		8 years 8 months 12 days
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net actuarial loss, Gross		$ (1,085)	$ (1,027)
Assumptions used to calculate net periodic benefit cost, service cost rate		3.79%
Actuarial loss, not subject to amortization		$ 444
Actuarial loss, subject to amortization		$ 641
Defined benefit plan, assumptions used calculating net periodic benefit cost, interest cost		3.42%
Expected return on plan assets		7.50%	7.50%
Rate of increase in future compensation levels for determining net periodic benefit cost		4.70%	4.70%
Net periodic benefit cost		$ 16	$ 79	$ 40
Estimated future employer contributions next fiscal year		115
Effect of 50 basis point increase on discount rate		19
Effect of 50 basis point increase on expected rate of return		17
Effect of 50 basis point increase on future compensation levels		1
Effect of 50 basis point decrease on discount rate		19
Effect of 50 basis point decrease on expected rate of return on plan assets		17
Effect of 50 basis point decrease on future compensation levels		1
Prior service costs (credits), Gross		$ (179)	$ (121)
Pension Benefits | Forecast
Pension Plan and Other Postretirement Benefits Plan
Net periodic benefit cost	$ 30