Retirement Benefits - Summary of Changes in Plan Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year	$ 13,571		$ 13,357
Actual return on plan assets	1,443		821
Employer contributions	332		339
Plan participants’ contributions	0		0
Benefits paid	(896)		(877)
Acquisitions/divestitures(a)	454		0
Settlements	(30)	[1]	(69)	[1]
Fair value of net plan assets at end of year	14,874		13,571
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of net plan assets at beginning of year	2,238		2,135
Actual return on plan assets	90		209
Employer contributions	291		83
Plan participants’ contributions	42		34
Benefits paid	(231)		(223)
Acquisitions/divestitures(a)	0		0
Settlements	0	[1]	0	[1]
Fair value of net plan assets at end of year	$ 2,430		$ 2,238

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