Equity Investments - NYX (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) shares
NYX
Schedule of Equity Method Investments [Line Items]
Investment cost	$ 91.9
Fair value as of December 31, 2017	90.4
Change in fair value loss (gain)	$ 1.5
NYX | Ordinary Shares
Schedule of Equity Method Investments [Line Items]
Number of shares issuable upon conversion (in shares) | shares	9,174,364
NYX | Class A Preferred
Schedule of Equity Method Investments [Line Items]
Number of Shares (in shares) | shares	40,000
NYX | Warrants
Schedule of Equity Method Investments [Line Items]
Warrants to acquire ordinary shares (in shares) | shares	3,535,178
NYX | Level 1 | Ordinary Shares
Schedule of Equity Method Investments [Line Items]
Number of Shares (in shares) | shares	39,000,000
Investment cost	$ 73.2
Fair value as of December 31, 2017	73.2
Change in fair value loss (gain)	0.0
NYX | Level 2 | Class A exchangeable preferred shares, warrants and other
Schedule of Equity Method Investments [Line Items]
Investment cost	18.7
Fair value as of December 31, 2017	17.2
Change in fair value loss (gain)	$ 1.5
NYX
Schedule of Equity Method Investments [Line Items]
Non-controlling interest acquired	36.00%