Nature of Operations and Significant Accounting Policies - Schedule of Retrospective Impact of Adoption (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe	$ 695	$ 725	$ 2,060	$ 2,271
Restructuring Charge	0	1	0	226
Restructuring Charge - Non-Operating	0	0	0	(70)
Other Income - Net	$ 19	14	$ 54	41
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe		717		2,249
Restructuring Charge		1		296
Restructuring Charge - Non-Operating		0		0
Other Income - Net		6		19
Reclass
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe		8		22
Restructuring Charge		0		(70)
Restructuring Charge - Non-Operating		0		(70)
Other Income - Net		$ 8		$ 22