Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Intangible assets:
Gross Carrying Amount	$ 1,555		$ 1,555		$ 1,541
Accumulated Amortization	(1,461)		(1,461)		(1,449)
Net Carrying Amount	94		94		92
Amortization expense	14	$ 18	29	$ 34
Customer lists and user base
Intangible assets:
Gross Carrying Amount	536		536		519
Accumulated Amortization	(460)		(460)		(445)
Net Carrying Amount	76		$ 76		$ 74
Weighted Average Useful Life (Years)			5 years		5 years
Marketing related
Intangible assets:
Gross Carrying Amount	578		$ 578		$ 584
Accumulated Amortization	(572)		(572)		(578)
Net Carrying Amount	6		$ 6		$ 6
Weighted Average Useful Life (Years)			5 years		5 years
Developed technologies
Intangible assets:
Gross Carrying Amount	280		$ 280		$ 278
Accumulated Amortization	(272)		(272)		(269)
Net Carrying Amount	8		$ 8		$ 9
Weighted Average Useful Life (Years)			3 years		3 years
All other
Intangible assets:
Gross Carrying Amount	161		$ 161		$ 160
Accumulated Amortization	(157)		(157)		(157)
Net Carrying Amount	$ 4		$ 4		$ 3
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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