Organization and Basis of Presentation (Details)	12 Months Ended
Dec. 31, 2018 segment item $ / shares	Mar. 01, 2019 $ / shares	Dec. 31, 2017 $ / shares
Number of reportable operating segments \| segment	4
Common stock, par value	$ 0.01		$ 0.01
Merger Agreement \| Subsequent event
Merger consideration per share		$ 30.30
Minimum
Number of countries in which execution services are offered \| item	50

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration