Retirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 326	$ 293	$ 317
Interest cost	710	749	650
Expected return on assets	(1,026)	(994)	(1,015)
Prior service cost (credit)	13	14	14
Actuarial loss	571	420	562
Settlement charges	2	2	9
Net periodic benefit cost	596	484	537
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	119	117	162
Interest cost	167	186	194
Expected return on assets	(151)	(154)	(132)
Prior service cost (credit)	(174)	(122)	(19)
Actuarial loss	80	50	83
Settlement charges	0	0	0
Net periodic benefit cost	$ 41	$ 77	$ 288