Nature of Operations and Significant Accounting Policies - Schedule of Retrospective Impact of Adoption (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe	$ 669	$ 751	$ 1,365	$ 1,546
Restructuring Charge	0	115	0	225
Restructuring Charge - Non-Operating	0	(7)	0	(70)
Other Income - Net	$ 18	14	$ 35	27
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe		743		1,532
Restructuring Charge		122		295
Restructuring Charge - Non-Operating		0		0
Other Income - Net		6		13
Reclass
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe		8		14
Restructuring Charge		(7)		(70)
Restructuring Charge - Non-Operating		(7)		(70)
Other Income - Net		$ 8		$ 14