Retirement Benefits and Trusteed Assets (Pension Plan - Pension Cost Inclusions) (Details) - Pension plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 92	$ 92	$ 100
Interest cost	214	219	210
Expected return on plan assets	(311)	(309)	(296)
Amortization of:
Net actuarial loss	176	164	205
Prior service cost	1	1	0
Other	0	0	2
Net pension cost	$ 172	$ 167	$ 221