Acquisitions - Schedule of Calculation of Purchase Consideration Transferred (Detail) - USD ($) $ in Millions			12 Months Ended
Jan. 12, 2016	Feb. 26, 2015	Dec. 31, 2016
Business Acquisition [Line Items]
Cash			$ 36.8
GFI Group Inc. [Member]
Business Acquisition [Line Items]
Cash	$ 111.2	$ 331.1	$ 89.9
Cost value of shares already owned (17,075,464 shares)		75.1
Redeemable noncontrolling interest (56,435,876 shares at $6.10 per share)		344.3
Total purchase consideration and noncontrolling interest (cost value)		750.5
Appreciation of shares already owned (17,075,464 shares at $6.10 per share less cost value)		29.0
Total purchase consideration and noncontrolling interest (fair value)		$ 779.5

Business combination consideration transferred appreciation of shares already owned.

No definition available.

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- Definition

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.

+ References

No definition available.

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- Definition

This element represents the fair value of the noncontrolling interest in the acquiree at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

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- Definition

Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Fair value at acquisition-date of the assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interests issued by the acquirer, including but not limited to, instruments or interests issued or issuable in consideration for the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6516133

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