GOODWILL AND INTANGIBLE ASSETS, NET - SCHEDULE OF INTANGIBLE ASSETS BY CATEGORY (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net	$ 1,996.0	$ 8.7
Customer relationships | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period (in years)	20 years	9 years
Market data customer relationships | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period (in years)	15 years	0 years
Technology | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period (in years)	7 years	4 years
Trademarks and tradenames | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period (in years)	2 years	6 years
Other | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period (in years)		2 years
Operating Segments | Options
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	$ (2.1)	$ (0.6)
Intangible assets, net	209.9	2.0
Operating Segments | Options | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	37.9	0.9
Operating Segments | Options | Market data customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	53.6	0.0
Operating Segments | Options | Technology
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	23.4	1.1
Operating Segments | Options | Trademarks and tradenames
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	1.4	0.4
Operating Segments | Options | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	0.2	0.2
Operating Segments | Options | Trading registrations and licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross	95.5	0.0
Operating Segments | U.S. Equities
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	(7.4)	0.0
Intangible assets, net	1,138.7	0.0
Operating Segments | U.S. Equities | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	222.9	0.0
Operating Segments | U.S. Equities | Market data customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	322.0	0.0
Operating Segments | U.S. Equities | Technology
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	22.5	0.0
Operating Segments | U.S. Equities | Trademarks and tradenames
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	6.0	0.0
Operating Segments | U.S. Equities | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	0.0	0.0
Operating Segments | U.S. Equities | Trading registrations and licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross	572.7	0.0
Operating Segments | European Equities
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	(2.3)	0.0
Intangible assets, net	415.7	0.0
Operating Segments | European Equities | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	160.7	0.0
Operating Segments | European Equities | Market data customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	60.3	0.0
Operating Segments | European Equities | Technology
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	22.6	0.0
Operating Segments | European Equities | Trademarks and tradenames
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	1.8	0.0
Operating Segments | European Equities | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	0.0	0.0
Operating Segments | European Equities | Trading registrations and licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross	172.6	0.0
Operating Segments | Global FX
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	(2.8)	0.0
Intangible assets, net	225.3	0.0
Operating Segments | Global FX | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	140.0	0.0
Operating Segments | Global FX | Market data customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	64.4	0.0
Operating Segments | Global FX | Technology
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	22.5	0.0
Operating Segments | Global FX | Trademarks and tradenames
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	1.2	0.0
Operating Segments | Global FX | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	0.0	0.0
Operating Segments | Global FX | Trading registrations and licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross	0.0	0.0
and Eliminations
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	(1.6)	(1.3)
Intangible assets, net	6.4	6.7
and Eliminations | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	3.0	3.0
and Eliminations | Market data customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	0.0	0.0
and Eliminations | Technology
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	4.0	4.0
and Eliminations | Trademarks and tradenames
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	1.0	1.0
and Eliminations | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	0.0	0.0
and Eliminations | Trading registrations and licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets, gross	$ 0.0	$ 0.0