Employee Pension and Other Postretirement Benefits - Additional Information (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
Employee pre-tax covered compensation percent		75.00%
Employee after-tax covered compensation percent		10.00%
Combined maximum employee contribution percent		75.00%
Employee pre-tax gross pay for which the employer contributes a match		4.00%
Contribution plan expenses		$ 48	47	37
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Service Cost Associated with Transferred Employees				6
Pension and other benefits adjustment, Gross		$ 906	906
Service Period required for eligibility		1 year
Service Period required for vesting		3 years
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax, Not Subject to Amortization		$ (444)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax, Subject to Amortization		(641)
Defined Benefit Plan, Future Amortization of Gain (Loss)		(76)
Net periodic benefit cost		16	$ 79	40
Estimated future employer contributions next fiscal year		$ 115
Assumptions used to calculate net periodic benefit cost, service cost rate		3.79%
Defined benefit plan, assumptions used calculating net periodic benefit cost, interest cost		3.42%
Expected return on plan assets		7.50%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.70%	4.70%
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
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Pension and other benefits adjustment, Gross		28	$ 67
Net periodic benefit cost		$ (4)	$ 7	(1)
Expected return on plan assets		7.50%	7.50%
Executive Supplemental Benefits Pension Plan
Pension Plan and Other Postretirement Benefits Plan
Accrued liability for ESBPs		$ 98	$ 101
Pension and other benefits adjustment, Gross		$ 40	37
Equity securities | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		63.00%
Equity securities | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		70.00%
Debt Securities | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		25.00%
Debt Securities | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		30.00%
Real estate | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Target plan asset allocation		12.00%
Minimum | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan Future Cost Sharing Percentage		25.00%
Maximum | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan Future Cost Sharing Percentage		50.00%
Recognized net actuarial gain (loss) | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 1,085	1,027	1,044	467
Recognized net actuarial gain (loss) | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		78	82	62	17
Amortization of prior service credit | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		(179)	(121)	(138)	0
Amortization of prior service credit | Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ (50)	$ (15)	$ (23)	$ 0
Forecast | Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Future Amortization of Gain (Loss)	$ (77)
Net periodic benefit cost	$ 30
Matching Range 1
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match		100.00%
Employee pre-tax gross pay for which the employer contributes a match		3.00%
Matching Range 2
Pension Plan and Other Postretirement Benefits Plan
Company pre-tax compensation match		50.00%
Employee pre-tax gross pay for which the employer contributes a match		2.00%