Held for Sale - Narrative (Details) £ in Millions, $ in Millions			9 Months Ended
	Dec. 11, 2015 USD ($) £ / $	Dec. 11, 2015 GBP (£) £ / $	Sep. 30, 2017 USD ($) £ / $	Oct. 27, 2017 USD ($) £ / $	Oct. 27, 2017 GBP (£) £ / $
TRAYPORT
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of voting interests acquired	100.00%	100.00%
Total purchase price	$ 620.0	£ 407
Consideration transferred in the purchase of common stock	$ 12.6
Foreign currency exchange rate | £ / $	1.5218	1.5218	1.3398
TRAYPORT | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Foreign currency translation loss			$ 74.0
TRAYPORT | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Foreign currency translation loss			74.0
Net assets			609.0
Carrying value of net assets			535.0
Total assets held for sale (included in prepaid expenses and other current assets)			595.0
Total liabilities held for sale (included in other current liabilities)			$ 60.0
Subsequent Event | TRAYPORT | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Foreign currency exchange rate | £ / $				1.3129	1.3129
Total assets held for sale (included in prepaid expenses and other current assets)				$ 722.0	£ 550