Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Revenues	$ 128,746	$ 120,769	$ 128,477	$ 131,484	$ 126,747	$ 114,531	$ 121,581	$ 120,835	$ 509,476	$ 483,694	$ 469,052
Net change in commissions and fees									274
Net change in recurring revenues									409
Net change in total revenues									683
Receivables related to contracts with customers	42,100				38,200				42,100	38,200
Contract assets	0								0
Impairment losses on receivables									0
Deferred Revenue	9,558				8,057				9,558	8,057
Revenue recognized included in deferred income at beginning of quarter									1,400
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenues:
Receivables related to contracts with customers	1,100								1,100
Deferred Revenue	600								600
Commissions and fees and recurring revenue
Revenues:
Revenues									502,800
Analytics
Revenues:
Revenues									43,108	45,249	$ 44,198
Deferred Revenue	$ 8,100				$ 6,900				$ 8,100	$ 6,900