Commitments and Contingencies - Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Minimum annual rental commitments, net of amounts due under subleases
Net amount due within next twelve months	$ 186
Net amount due in year two	154
Net amount due in year three	108
Net amount due in year four	77
Net amount due in year five	55
Net amount due thereafter	266
Rent expense	$ 579	$ 360	$ 200