Nature of Operations and Significant Accounting Policies - Schedule of Retrospective Impact of Adoption (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe	$ 696	$ 795
Restructuring Charge	0	110
Restructuring Charge - Non-Operating	0	(63)
Other Income - Net	$ 17	13
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe		789
Restructuring Charge		173
Restructuring Charge - Non-Operating		0
Other Income - Net		7
Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Labor and Fringe		6
Restructuring Charge		(63)
Restructuring Charge - Non-Operating		(63)
Other Income - Net		$ 6