Summary of Significant Accounting Policies - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Support and maintenance revenue recognition period	3 years
Term of the maintenance contract	3 years
Maximum
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Support and maintenance revenue recognition period	5 years
Term of the maintenance contract	5 years