Employee Benefit Plans - Fair Value of Plan Assets and Funded Status (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined benefit plan, actuarial losses (gains)	$ 3,300
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets Period Start	9,880
Defined Benefit Plan, Fair Value of Plan Assets period End	10,351
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined benefit plan, benefit obligation period start	19,060	[1]	21,489
Defined benefit plan, service cost	0		0		0
Defined benefit plan, interest cost	928		861		930
Defined benefit plan, actuarial losses (gains)	2,923		(2,212)
Defined benefit plan, benefits paid, including lump sum and annuities	(1,055)		(1,078)
Defined benefit plan, participant contributions	0		0
Defined benefit plan, benefit obligation period end	21,856	[1]	19,060	[1]	21,489
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets Period Start	8,937		8,196
Defined Benefit Plan, Actual Return on Plan Assets	556		905
Defined Benefit Plan, Contributions by Employer	917		914
Defined benefit plan, participant contributions	0		0
Defined benefit plan, benefits paid, including lump sum and annuities	(1,055)		(1,078)
Defined Benefit Plan, Fair Value of Plan Assets period End	9,355		8,937		8,196
Defined Benefit Plan, Funded Status of Plan [Abstract]
Defined Benefit Plan, Funded Status of Plan	(12,501)		(10,123)
Other Postretirement and Postemployment Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined benefit plan, benefit obligation period start	3,205	[1]	3,582
Defined benefit plan, service cost	52		49		56
Defined benefit plan, interest cost	155		143		164
Defined benefit plan, actuarial losses (gains)	338		(301)
Defined benefit plan, benefits paid, including lump sum and annuities	(307)		(313)
Defined benefit plan, participant contributions	44		45
Defined benefit plan, benefit obligation period end	3,487	[1]	3,205	[1]	3,582
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Fair Value of Plan Assets Period Start	1,043		1,004
Defined Benefit Plan, Actual Return on Plan Assets	57		129
Defined Benefit Plan, Contributions by Employer	160		191
Defined benefit plan, participant contributions	44		45
Defined benefit plan, benefits paid, including lump sum and annuities	(307)		(313)
Defined Benefit Plan, Fair Value of Plan Assets, Period Increase (Decrease)	(322)		(326)
Defined Benefit Plan, Fair Value of Plan Assets period End	982		1,043		1,004
Defined Benefit Plan, Funded Status of Plan [Abstract]
Defined Benefit Plan, Funded Status of Plan	$ (2,505)		$ (2,162)

[1] At the end of each year presented, our accumulated benefit obligations for our pension plans are equal to the benefit obligations shown above.