Summary of Significant Accounting Policies (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Loss
Unamortized benefit plan costs, net of tax benefit of $3,056 for 2017 and $3,439 for 2016	$ (4,586)	$ (5,416)
Cumulative translation adjustment	(136)	(132)
Net unrealized gain on marketable securities and cash flow hedges, net of tax	4	2
Total accumulated other comprehensive loss	(4,718)	(5,546)
Unamortized benefit plan costs - Tax Benefit	3,056	3,439
Unamortized benefit plan, actuarial loss, net of tax	4,700	5,600
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	$ 398	$ 402	$ 388