Summary of Significant Accounting Policies - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2017
Segment Reporting Information [Line Items]
Maximum scheduled period to commence employment for search revenue recognition	30 days
Typical contingency period for occurrence of fallouts	90 days
Government Solutions
Segment Reporting Information [Line Items]
Percentage of consolidated revenue	8.00%
Time and material contracts revenue as percentage of aggregate segment revenue	58.00%
Fixed-price contracts revenue recognized as percentage of aggregate segment revenue	30.00%
Product contracts revenue recognized as percentage of aggregate segment revenue	12.00%