Revenue Recognition - Narrative (Details) - obligation	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Revenue Recognition and Deferred Revenue [Abstract]
Number of performance obligations	2
Number of performance obligations related to trade execution	1
Period of recognition of performance obligations		2 years
Period of customer revenue list recognized for NYSE	9 years
Period of customer revenue list recognized for NYSE area and NYSE American		5 years
Period of customer revenue list recognized for NYSE, modifications		6 years
Period of customer revenue list recognized for NYSE area and NYSE American, modifications		3 years