Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Intangible assets:
Gross Carrying Amount	$ 1,457		$ 1,457		$ 1,541
Accumulated Amortization	(1,378)		(1,378)		(1,449)
Net Carrying Amount	79		79		92
Amortization expense	13	$ 15	42	$ 49
Customer lists and user base
Intangible assets:
Gross Carrying Amount	496		496		519
Accumulated Amortization	(431)		(431)		(445)
Net Carrying Amount	65		$ 65		$ 74
Weighted Average Useful Life (Years)			5 years		5 years
Marketing related
Intangible assets:
Gross Carrying Amount	529		$ 529		$ 584
Accumulated Amortization	(523)		(523)		(578)
Net Carrying Amount	6		$ 6		$ 6
Weighted Average Useful Life (Years)			5 years		5 years
Developed technologies
Intangible assets:
Gross Carrying Amount	272		$ 272		$ 278
Accumulated Amortization	(266)		(266)		(269)
Net Carrying Amount	6		$ 6		$ 9
Weighted Average Useful Life (Years)			3 years		3 years
All other
Intangible assets:
Gross Carrying Amount	160		$ 160		$ 160
Accumulated Amortization	(158)		(158)		(157)
Net Carrying Amount	$ 2		$ 2		$ 3
Weighted Average Useful Life (Years)			4 years		4 years