For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Long-lived assets other than financial instruments, long-term customer relationships of a financial institution, mortgage and other servicing rights, deferred policy acquisition costs, and deferred tax assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Topic 280
-SubTopic 10
-Section 50
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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Segment and Geographic Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 1,589	$ 1,513	$ 1,509	$ 1,453	$ 1,399	$ 1,439	$ 1,482	$ 1,341	$ 6,063	$ 5,661	$ 5,313
Long-lived Assets	4,734				4,780				4,734	4,780
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									3,658	3,461	3,202
Long-lived Assets	4,285				4,335				4,285	4,335
European region
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,473	1,330	1,265
Long-lived Assets	346				341				346	341
Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									594	575	566
Long-lived Assets	54				58				54	58
Rest of the world
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									338	295	$ 280
Long-lived Assets	$ 49				$ 46				$ 49	$ 46
Geographic Concentration Risk \| Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									100.00%	100.00%	100.00%
Geographic Concentration Risk \| Revenue \| U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									60.00%	61.00%	60.00%
Geographic Concentration Risk \| Revenue \| European region
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									24.00%	24.00%	24.00%
Geographic Concentration Risk \| Revenue \| Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									10.00%	10.00%	11.00%
Geographic Concentration Risk \| Revenue \| Rest of the world
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									6.00%	5.00%	5.00%
Geographic Concentration Risk \| Long-lived Assets
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									100.00%	100.00%
Geographic Concentration Risk \| Long-lived Assets \| U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									91.00%	91.00%
Geographic Concentration Risk \| Long-lived Assets \| European region
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									7.00%	7.00%
Geographic Concentration Risk \| Long-lived Assets \| Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									1.00%	1.00%
Geographic Concentration Risk \| Long-lived Assets \| Rest of the world
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk									1.00%	1.00%