Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Intangible assets:
Gross Carrying Amount	$ 1,549		$ 1,549		$ 1,494
Accumulated Amortization	(1,443)		(1,443)		(1,425)
Intangible Assets, Net	106		106		69
Amortization expense	15	$ 16	49	$ 48
Customer lists and user base
Intangible assets:
Gross Carrying Amount	522		522		458
Accumulated Amortization	(441)		(441)		(430)
Intangible Assets, Net	81		$ 81		$ 28
Weighted Average Useful Life (Years)			5 years		5 years
Marketing related
Intangible assets:
Gross Carrying Amount	588		$ 588		$ 607
Accumulated Amortization	(579)		(579)		(587)
Intangible Assets, Net	9		$ 9		$ 20
Weighted Average Useful Life (Years)			5 years		5 years
Developed technologies
Intangible assets:
Gross Carrying Amount	279		$ 279		$ 273
Accumulated Amortization	(268)		(268)		(258)
Intangible Assets, Net	11		$ 11		$ 15
Weighted Average Useful Life (Years)			3 years		3 years
All other
Intangible assets:
Gross Carrying Amount	160		$ 160		$ 156
Accumulated Amortization	(155)		(155)		(150)
Intangible Assets, Net	$ 5		$ 5		$ 6
Weighted Average Useful Life (Years)			4 years		4 years

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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