Goodwill, Trade Names, and Other Intangible Assets - Acquired Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 110,068	$ 80,168
Accumulated Amortization	49,092	43,333
Net Carrying Amount	60,976	36,835
Trade names	26,702	35,402
Intangible assets not subject to amortization, net	87,678	72,237
Databases
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	42,909	31,609
Accumulated Amortization	18,702	16,147
Net Carrying Amount	24,207	15,462
Customer relationships
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	55,524	41,724
Accumulated Amortization	25,912	23,316
Net Carrying Amount	29,612	18,408
Non-compete agreements
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3,919	3,619
Accumulated Amortization	3,600	3,527
Net Carrying Amount	319	92
Trade names
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,716	3,216
Accumulated Amortization	878	343
Net Carrying Amount	$ 6,838	$ 2,873