Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Intangible assets:
Gross Carrying Amount	$ 1,550		$ 1,550		$ 1,494
Accumulated Amortization	(1,427)		(1,427)		(1,425)
Intangible Assets, Net	123		123		69
Amortization expense	18	$ 16	34	$ 32
Customer lists and user base
Intangible assets:
Gross Carrying Amount	523		523		458
Accumulated Amortization	(431)		(431)		(430)
Intangible Assets, Net	92		$ 92		$ 28
Weighted Average Useful Life (Years)			5 years		5 years
Marketing related
Intangible assets:
Gross Carrying Amount	590		$ 590		$ 607
Accumulated Amortization	(577)		(577)		(587)
Intangible Assets, Net	13		$ 13		$ 20
Weighted Average Useful Life (Years)			5 years		5 years
Developed technologies
Intangible assets:
Gross Carrying Amount	279		$ 279		$ 273
Accumulated Amortization	(266)		(266)		(258)
Intangible Assets, Net	13		$ 13		$ 15
Weighted Average Useful Life (Years)			3 years		3 years
All other
Intangible assets:
Gross Carrying Amount	158		$ 158		$ 156
Accumulated Amortization	(153)		(153)		(150)
Intangible Assets, Net	$ 5		$ 5		$ 6
Weighted Average Useful Life (Years)			4 years		4 years