Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Customer listing fee recognition, estimated service period (up to)	9 years
Accounting Standards Update 2017-07, Net Periodic Benefit Cost
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net periodic benefit (expense)	$ (2)	$ 2

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant