Intangible Assets - Intangible Assets Subject To Amortization (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]
Weighted-Average Estimated Useful Life (years)	9 years 9 months 50 days
Gross Carrying Amount	$ 172,707	$ 208,270
Accumulated Amortization	$ 98,474	104,659
Technology
Indefinite-lived Intangible Assets [Line Items]
Weighted-Average Estimated Useful Life (years)	7 years 10 months 24 days
Gross Carrying Amount	$ 25,563	25,563
Accumulated Amortization	$ 21,430	19,020
Relationships with financial advisors
Indefinite-lived Intangible Assets [Line Items]
Weighted-Average Estimated Useful Life (years)	14 years 5 months 12 days
Gross Carrying Amount	$ 123,477	117,995
Accumulated Amortization	$ 56,953	49,925
Covenants not-to-compete
Indefinite-lived Intangible Assets [Line Items]
Weighted-Average Estimated Useful Life (years)	3 years 8 months 55 days
Gross Carrying Amount	$ 6,752	6,421
Accumulated Amortization	$ 6,176	5,732
Trade names
Indefinite-lived Intangible Assets [Line Items]
Weighted-Average Estimated Useful Life (years)	7 years 7 months 50 days
Gross Carrying Amount	$ 16,915	16,910
Accumulated Amortization	$ 13,915	12,245
Renewal revenue
Indefinite-lived Intangible Assets [Line Items]
Weighted-Average Estimated Useful Life (years)	7 years 10 months 24 days
Gross Carrying Amount	$ 0	41,381
Accumulated Amortization	$ 0	$ 17,737