Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($)
Changes in goodwill
Balance at the beginning of the period	$ 836,583
Goodwill recognized in ITG Acquisition	312,343
Balance at the end of the period	1,148,926
Operating Segments \| Market Making
Changes in goodwill
Balance at the beginning of the period	755,292
Goodwill recognized in ITG Acquisition	0
Balance at the end of the period	755,292
Operating Segments \| Execution Services
Changes in goodwill
Balance at the beginning of the period	81,291
Goodwill recognized in ITG Acquisition	312,343
Balance at the end of the period	393,634
Corporate
Changes in goodwill
Balance at the beginning of the period	0
Goodwill recognized in ITG Acquisition	0
Balance at the end of the period	$ 0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
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-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

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+ References

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