Retirement Benefits - Summary of Changes in Benefit Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	$ 15,459		$ 16,800
Service cost	293		317		280
Interest cost	749		650		698
Plan participants’ contributions	0		0
Actuarial loss (gain)	2,095		(1,363)
Plan amendments	0		1
Acquisitions/divestitures(a)	594		0
Curtailments	(8)		0
Settlements	(30)	[1]	(69)	[1]
Gross benefits paid	(896)		(877)
Net benefit obligation at end of year	18,256		15,459		16,800
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Net benefit obligation at beginning of year	4,451		4,820
Service cost	117		162		156
Interest cost	186		194		205
Plan participants’ contributions	42		34
Actuarial loss (gain)	502		(551)
Plan amendments	(1,012)		15
Acquisitions/divestitures(a)	142		0
Curtailments	0		0
Settlements	0	[1]	0	[1]
Gross benefits paid	(231)		(223)
Net benefit obligation at end of year	$ 4,197		$ 4,451		$ 4,820

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