Acquisition - Narrative (Details) $ in Thousands		12 Months Ended
	Apr. 01, 2015 USD ($) Loan shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 30, 2015 USD ($) Loan	Mar. 31, 2015 USD ($)
Business Acquisition [Line Items]
Payments to acquire Cogent Partners, LP		$ 0	$ 0	$ 45,265
Bank term loans payable		0	16,875
Revenue target for the Earnout	$ 80,000
Contingent obligation due selling unitholders of Cogent		13,763	15,095
Increase (decrease) in fair value of contingent obligation		(1,332)	1,448	503
Goodwill		217,737	208,186	209,024
Cogent Partners, LP
Business Acquisition [Line Items]
Ownership percent acquired	100.00%
Payments to acquire Cogent Partners, LP	$ 44,000
Common stock paid at acquisition closing (shares) | shares	779,454
Contingent consideration, cash	$ 18,900
Possible contingent consideration (shares) | shares	334,048
Number of bank term loan facilities (loan) | Loan	2				2
Bank term loans payable	$ 45,000				$ 45,000
Distribution period ending on the second anniversary of the closing	2 years
Distribution period ending on the fourth anniversary of the closing	2 years
Fair value of contingent common shares	$ 11,900
Contingent obligation due selling unitholders of Cogent	13,100	13,800
Increase (decrease) in fair value of contingent obligation		(1,300)	1,500	500
Transaction costs				1,200
Purchase price	100,000
Goodwill	93,116
Amortization expense		$ 300	500	500
Amount to trigger post-closing distribution payment						$ 5,000
Post closing distribution			$ 8,400	7,900
Escrow deposit	8,900
Portion of escrow returned to company				$ 500
Goodwill useful life		15 years
Ratio of compensation to total revenue				54.00%
Cogent Partners, LP | Minimum
Business Acquisition [Line Items]
Intangible assets useful life		1 year
Cogent Partners, LP | Maximum
Business Acquisition [Line Items]
Intangible assets useful life		3 years
Cogent Partners, LP | Short-term debt | Cogent 12 Month Facility
Business Acquisition [Line Items]
Bank term loans payable	$ 22,500				$ 22,500