Basis of Presentation and Significant Accounting Policies - Total Lease Costs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Operating lease cost	$ 100	$ 200
Variable lease cost	59	118
Sublease income	(11)	(21)
Total lease cost	$ 148	$ 297