Summary of Significant Accounting Policies - Transaction Revenues Narrative (Details)	12 Months Ended
	Dec. 31, 2017 subscription_arrangement_type
Property, Plant and Equipment [Line Items]
Types of subscription arrangement	3
Minimum
Property, Plant and Equipment [Line Items]
Subscription fee arrangement term	1 month
Maximum
Property, Plant and Equipment [Line Items]
Subscription fee arrangement term	6 months