Revenue Recognition Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total Revenues	$ 1,182.3	$ 1,272.7	$ 2,704.4	$ 2,452.3
Services transferred at a point in time [Member]
Revenues	881.3	988.1	2,092.5	1,881.0
Services transferred over time [Member]
Revenues	298.8	281.9	606.3	559.6
One-time charges and miscellaneous revenues [Member]
Revenues	2.2	2.7	5.6	11.7
Clearing and transaction fees [Member]
Revenues	940.2	1,051.8	2,219.0	2,004.4
Clearing and transaction fees [Member] | Interest rate [Member]
Revenues	221.4	347.4	639.7	650.2
Clearing and transaction fees [Member] | Equities [Member]
Revenues	201.3	148.1	449.5	294.0
Clearing and transaction fees [Member] | Foreign Exchange [Member]
Revenues	35.9	39.3	84.1	80.5
Clearing and transaction fees [Member] | Agricultural commodity [Member]
Revenues	108.7	141.5	226.4	246.5
Clearing and transaction fees [Member] | Energy [Member]
Revenues	194.0	179.3	415.8	344.3
Clearing and transaction fees [Member] | Metal [Member]
Revenues	49.6	58.1	128.4	109.1
Clearing and transaction fees [Member] | Cash Markets [Member]
Revenues	112.4	120.7	236.8	243.6
Clearing and transaction fees [Member] | Interest Rate Swap [Member]
Revenues	16.9	17.4	38.3	36.2
Market data and information services
Revenues	134.7	128.3	266.2	258.4
Other
Revenues	$ 107.4	$ 92.6	$ 219.2	$ 189.5