Retirement Plans (Narrative) (Details) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) plan shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($)
Compensation And Retirement Disclosure Line Items
Defined contribution plan, cost recognized	$ 240	$ 238	$ 204
Common stock, shares held in employee trust, shares | shares	24	25
Percentage of defined contribution plans held from outstanding shares	8.00%	7.00%
Number Of Noncontributory Defined Benefit Pension Plans Covering Government Business Employees | plan	6
Number Of Contributory Defined Benefit Pension Plans Covering Government Business Employees | plan	6
Number Of Noncontributory Defined Benefit Pension Plans Covering Commercial Business Employees | plan	2
Number Of Contributory Defined Benefit Pension Plans Covering Commercial Business Employees | plan	4
Number of non-qualified supplemental executive plans | plan	1
Period in which the difference between the actual and expected return on plan assets for qualified plans are recognized in years	5 years
Defined benefit plan, effect of one percentage point increase on service and interest cost components	$ 6
Defined benefit plan, effect of one percentage point decrease on service and interest cost components	(5)
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	82
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	$ (65)
Percentage of pension plan assets held in a single trust for primary domestic government and commercial pension plans	90.00%
Pension Benefits
Compensation And Retirement Disclosure Line Items
Defined Benefit Plan, Actuarial Gain (Loss)	$ 685	$ (2,083)
Defined Benefit Plan, Contributions by Employer	187	513
Expected contribution to defined benefit contribution plans	200
Defined benefit plan, accumulated benefit obligation	12,200	12,800
Other Post-retirement Benefits
Compensation And Retirement Disclosure Line Items
Defined Benefit Plan, Actuarial Gain (Loss)	104	(30)
Defined Benefit Plan, Contributions by Employer	$ 0	$ 6