Acquisition - Purchase Price Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2015
Business Acquisition [Line Items]
Goodwill	$ 217,737	$ 208,186	$ 209,024
Cogent Partners, LP
Business Acquisition [Line Items]
Current assets				$ 13,970
Property and equipment				599
Other assets				651
Identifiable intangible assets				1,300
Goodwill				93,116
Total assets				109,636
Current liabilities				9,640
Total liabilities				9,640
Net assets				$ 99,996